Channel access method for data transmission, and wireless communication method and wireless communication terminal using same

ABSTRACT

The present invention relates to a channel access method for data transmission, and a wireless communication method and a wireless communication terminal using the same, and more particularly, to a wireless communication method and a wireless communication terminal for efficiently scheduling data transmissions of a plurality of terminals. To this end, provided are a wireless communication terminal comprising a processor and a transceiver, wherein the processor receives a trigger frame indicating an uplink multi-user transmission, and transmits an uplink multi-user PLCP protocol data unit (PPDU) in response to the received trigger frame, wherein the trigger frame and a non-legacy preamble of the uplink multi-user PPDU contains remaining transmission opportunity (TXOP) time information of a current TXOP and a wireless communication method using the same.

TECHNICAL FIELD

The present invention relates to a channel access method for datatransmission, and a wireless communication method and a wirelesscommunication terminal using the same, and more particularly, to awireless communication method and a wireless communication terminal forefficiently scheduling data transmissions of a plurality of terminals.

BACKGROUND ART

In recent years, with supply expansion of mobile apparatuses, a wirelessLAN technology that can provide a rapid wireless Internet service to themobile apparatuses has been significantly spotlighted. The wireless LANtechnology allows mobile apparatuses including a smart phone, a smartpad, a laptop computer, a portable multimedia player, an embeddedapparatus, and the like to wirelessly access the Internet in home or acompany or a specific service providing area based on a wirelesscommunication technology in a short range.

Institute of Electrical and Electronics Engineers (IEEE) 802.11 hascommercialized or developed various technological standards since aninitial wireless LAN technology is supported using frequencies of 2.4GHz. First, the IEEE 802.11b supports a communication speed of a maximumof 11 Mbps while using frequencies of a 2.4 GHz band. IEEE 802.11a whichis commercialized after the IEEE 802.11b uses frequencies of not the 2.4GHz band but a 5 GHz band to reduce an influence by interference ascompared with the frequencies of the 2.4 GHz band which aresignificantly congested and improves the communication speed up to amaximum of 54 Mbps by using an OFDM technology. However, the IEEE802.11a has a disadvantage in that a communication distance is shorterthan the IEEE 802.11b. In addition, IEEE 802.11g uses the frequencies ofthe 2.4 GHz band similarly to the IEEE 802.11b to implement thecommunication speed of a maximum of 54 Mbps and satisfies backwardcompatibility to significantly come into the spotlight and further, issuperior to the IEEE 802.11a in terms of the communication distance.

Moreover, as a technology standard established to overcome a limitationof the communication speed which is pointed out as a weak point in awireless LAN, IEEE 802.11n has been provided. The IEEE 802.11n aims atincreasing the speed and reliability of a network and extending anoperating distance of a wireless network. In more detail, the IEEE802.11n supports a high throughput (HT) in which a data processing speedis a maximum of 540 Mbps or more and further, is based on a multipleinputs and multiple outputs (MIMO) technology in which multiple antennasare used at both sides of a transmitting unit and a receiving unit inorder to minimize a transmission error and optimize a data speed.Further, the standard can use a coding scheme that transmits multiplecopies which overlap with each other in order to increase datareliability.

As the supply of the wireless LAN is activated and further, applicationsusing the wireless LAN are diversified, the need for new wireless LANsystems for supporting a higher throughput (very high throughput (VHT))than the data processing speed supported by the IEEE 802.11n has comeinto the spotlight. Among them, IEEE 802.11ac supports a wide bandwidth(80 to 160 MHz) in the 5 GHz frequencies. The IEEE 802.11ac standard isdefined only in the 5 GHz band, but initial 11ac chipsets will supporteven operations in the 2.4 GHz band for the backward compatibility withthe existing 2.4 GHz band products. Theoretically, according to thestandard, wireless LAN speeds of multiple stations are enabled up to aminimum of 1 Gbps and a maximum single link speed is enabled up to aminimum of 500 Mbps. This is achieved by extending concepts of awireless interface accepted by 802.11n, such as a wider wirelessfrequency bandwidth (a maximum of 160 MHz), more MIMO spatial streams (amaximum of 8), multi-user MIMO, and high-density modulation (a maximumof 256 QAM). Further, as a scheme that transmits data by using a 60 GHzband instead of the existing 2.4 GHz/5 GHz, IEEE 802.11ad has beenprovided. The IEEE 802.11ad is a transmission standard that provides aspeed of a maximum of 7 Gbps by using a beamforming technology and issuitable for high bit rate moving picture streaming such as massive dataor non-compression HD video. However, since it is difficult for the 60GHz frequency band to pass through an obstacle, it is disadvantageous inthat the 60 GHz frequency band can be used only among devices in ashort-distance space.

Meanwhile, in recent years, as next-generation wireless LAN standardsafter the 802.11ac and 802.11ad, discussion for providing ahigh-efficiency and high-performance wireless LAN communicationtechnology in a high-density environment is continuously performed. Thatis, in a next-generation wireless LAN environment, communication havinghigh frequency efficiency needs to be provided indoors/outdoors underthe presence of high-density stations and access points (APs) andvarious technologies for implementing the communication are required.

DISCLOSURE Technical Problem

The present invention has an object to providehigh-efficiency/high-performance wireless LAN communication in ahigh-density environment as described above.

In addition, the present invention has an object to maximize resourceutilization efficiency by performing efficient scheduling of an uplinkmulti-user transmission process.

In addition, the present invention has an object to control operationsof terminals not participating in a transmission process in the uplinkmulti-user transmission process.

Technical Solution

In order to achieve the objects, the present invention provides awireless communication method and a wireless communication terminal asbelow.

First, an exemplary embodiment of the present invention provides awireless communication terminal for transmitting data through amulti-channel including a primary channel and at least one secondarychannel, wherein the wireless communication terminal includes aprocessor and a transceiver, and wherein the processor transmits a firstdata through the multi-channel via the transceiver, performs a backoffprocedure when the primary channel is idle for a predetermined firsttime after the transmission of the first data is completed, and switchesto a transmission standby state when when a backoff counter of thebackoff procedure expires.

The processor may perform a first CCA procedure of the secondary channelfor a predetermined second time before the backoff counter of thebackoff procedure expires, and switch to the transmission standby statewhen the secondary channel is idle for the second time.

The processor may obtain a new backoff counter and perform a backoffprocedure for the transmission standby stated using the new backoffcounter when at least one secondary channel in which the first CCAprocedure is performed is busy.

The predetermined first time may be an AIFS.

The second time may be longer than a PIFS.

When a second data to be transmitted occurs in the transmission standbystate, the terminal may attempt to transmit the second data afterperforming a second CCA procedure of the primary channel and thesecondary channel for a predetermined time without performing a separatebackoff procedure.

The second CCA procedure of the secondary channel may be performed for alonger time than a PIFS.

The processor may obtain a new backoff counter and perform a backoffprocedure for a transmission of the second data using the new backoffcounter when at least one secondary channel is busy during the secondCCA procedure of the primary channel and the secondary channel.

In addition, an exemplary embodiment of the present invention provides awireless communication method of a wireless communication terminal fortransmitting data through a multi-channel including a primary channeland at least one secondary channel, including: transmitting a first datathrough the multi-channel via a transceiver; performing a backoffprocedure when the primary channel is idle for a predetermined firsttime after the transmission of the first data is completed; andswitching to a transmission standby state when a backoff counter of thebackoff procedure expires.

Next, another exemplary embodiment of the present invention provides awireless communication terminal including a processor and a transceiver,wherein the processor receives a trigger frame indicating an uplinkmulti-user transmission, and transmits an uplink multi-user PLCPprotocol data unit (PPDU) in response to the received trigger frame,wherein the trigger frame and a non-legacy preamble of the uplinkmulti-user PPDU contains remaining transmission opportunity (TXOP) timeinformation of a current TXOP.

The remaining TXOP time information may be represented by apredetermined TXOP duration field of a high efficiency signal field A(HE-SIG-A) of the non-legacy preamble.

The TXOP duration field may consist of fewer bits than a TXOP field of aMAC header of a corresponding packet.

The TXOP duration field may indicate the remaining TXOP time informationin a symbol unit.

The remaining TXOP time information may be represented based on acombination of a Length field and a Rate field of a legacy preamble, anda predetermined field of the non-legacy preamble of a correspondingpacket.

An uplink multi-user transmission non-participating terminal that hasreceived at least one of the trigger frame and the uplink multi-userPPDU may set a network allocation vector (NAV) based on the remainingTXOP time information.

When an additional uplink multi-user transmission process is performedwithin the current TXOP, an M-STA BA corresponding to the uplinkmulti-user PPDU and a next trigger frame indicating the additionaluplink multi-user transmission may be aggregated and transmitted in asingle A-MPDU.

In addition, another exemplary embodiment of the present inventionprovides a wireless communication method of a wireless communicationterminal, including: receiving a trigger frame indicating an uplinkmulti-user transmission, and transmitting an uplink multi-user PLCPprotocol data unit (PPDU) in response to the received trigger frame,wherein the trigger frame and a non-legacy preamble of the uplinkmulti-user PPDU contains remaining transmission opportunity (TXOP) timeinformation of a current TXOP.

Advantageous Effects

According to the embodiment of the present invention, the reliability ofthe uplink multi-user transmission can be ensured and the performancethereof can be improved through efficient scheduling.

According to an embodiment of the present invention, an efficientchannel access method of each terminal is provided through a postbackoff procedure in a multi-channel uplink multi-user transmission.

According to another embodiment of the present invention, by insertingTXOP information into a preamble of a non-legacy packet, neighboringterminals can obtain TXOP information and set a NAV in early stages.

According to an embodiment of the present invention, it is possible toincrease the total resource utilization rate in the contention-basedchannel access system and improve the performance of the wireless LANsystem.

DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a wireless LAN system according to an embodiment ofthe present invention.

FIG. 2 illustrates a wireless LAN system according to another embodimentof the present invention.

FIG. 3 illustrates a configuration of a station according to anembodiment of the present invention.

FIG. 4 illustrates a configuration of an access point according to anembodiment of the present invention.

FIG. 5 schematically illustrates a process in which a STA and an AP seta link.

FIG. 6 illustrates a carrier sense multiple access (CSMA)/collisionavoidance (CA) method used in wireless LAN communication.

FIG. 7 illustrates a method for performing a distributed coordinationfunction (DCF) using a request to send (RTS) frame and a clear to send(CTS) frame.

FIG. 8 illustrates a channel access method for a multi-channeldownlink/uplink transmission according to an embodiment of the presentinvention.

FIG. 9 illustrates a channel access method for a multi-channel uplinkmulti-user transmission according to an embodiment of the presentinvention.

FIG. 10 illustrates a channel access method for a multi-channel uplinkmulti-user transmission according to another embodiment of the presentinvention.

FIG. 11 illustrates a multi-channel uplink multi-user transmissionmethod according to yet another embodiment of the present invention.

FIG. 12 illustrates a periodic uplink multi-user data transmissionmethod according to a further embodiment of the present invention.

FIG. 13 illustrates an uplink multi-user transmission process accordingto the embodiment of the present invention.

FIG. 14 illustrates an embodiment of a placement situation of terminalsaround a specific BSS.

FIG. 15 illustrates an uplink multi-user transmission process accordingto an embodiment of the present invention and operations of hidden nodesaccording to the process.

FIG. 16 illustrates an uplink multi-user transmission process accordingto another embodiment of the present invention and operations of hiddennodes according to the process.

FIG. 17 illustrates an uplink multi-user transmission process accordingto yet another embodiment of the present invention and operations ofhidden nodes according to the process.

FIG. 18 illustrates a hidden node protection method in a multi-usertransmission process.

FIG. 19 illustrates various embodiments of an MPDU format of an MU-RTS.

FIG. 20 illustrates a method of supporting data transmission/receptionof an outdoor terminal using MU-RTS and CTS.

FIG. 21 illustrates a further embodiment of a downlink multi-usertransmission process.

FIG. 22 illustrates a further embodiment of an uplink multi-usertransmission process.

FIG. 23 illustrates an uplink multi-user transmission process accordingto a further embodiment of the present invention and operations ofhidden nodes according to the process.

DETAILED DESCRIPTION OF THE INVENTION

Terms used in the specification adopt general terms which are currentlywidely used by considering functions in the present invention, but theterms may be changed depending on an intention of those skilled in theart, customs, and emergence of new technology. Further, in a specificcase, there is a term arbitrarily selected by an applicant and in thiscase, a meaning thereof will be described in a corresponding descriptionpart of the invention. Accordingly, it should be revealed that a termused in the specification should be analyzed based on not just a name ofthe term but a substantial meaning of the term and contents throughoutthe specification.

Throughout this specification and the claims that follow, when it isdescribed that an element is “coupled” to another element, the elementmay be “directly coupled” to the other element or “electrically coupled”to the other element through a third element. Further, unless explicitlydescribed to the contrary, the word “comprise” and variations such as“comprises” or “comprising”, will be understood to imply the inclusionof stated elements but not the exclusion of any other elements.Moreover, limitations such as “or more” or “or less” based on a specificthreshold may be appropriately substituted with “more than” or “lessthan”, respectively.

This application claims priority to and the benefit of Korean PatentApplication Nos. 10-2015-0092531, 10-2015-0098711 and 10-2015-0100686filed in the Korean Intellectual Property Office and the embodiments andmentioned items described in the respective application, which forms thebasis of the priority, shall be included in the Detailed Description ofthe present application.

FIG. 1 is a diagram illustrating a wireless LAN system according to anembodiment of the present invention. The wireless LAN system includesone or more basic service sets (BSS) and the BSS represents a set ofapparatuses which are successfully synchronized with each other tocommunicate with each other. In general, the BSS may be classified intoan infrastructure BSS and an independent BSS (IBSS) and FIG. 1illustrates the infrastructure BSS between them.

As illustrated in FIG. 1 , the infrastructure BSS (BSS1 and BSS2)includes one or more stations STA1, STA2, STA3, STA4, and STA5, accesspoints PCP/AP-1 and PCP/AP-2 which are stations providing a distributionservice, and a distribution system (DS) connecting the multiple accesspoints PCP/AP-1 and PCP/AP-2.

The station (STA) is a predetermined device including medium accesscontrol (MAC) following a regulation of an IEEE 802.11 standard and aphysical layer interface for a wireless medium, and includes both anon-access point (non-AP) station and an access point (AP) in a broadsense. Further, in the present specification, a term ‘terminal’ may beused to refer to a non-AP STA, or an AP, or to both terms. A station forwireless communication includes a processor and a transceiver andaccording to the embodiment, may further include a user interface unitand a display unit. The processor may generate a frame to be transmittedthrough a wireless network or process a frame received through thewireless network and besides, perform various processing for controllingthe station. In addition, the transceiver is functionally connected withthe processor and transmits and receives frames through the wirelessnetwork for the station. According to the present invention, a terminalmay be used as a term which includes user equipment (UE).

The access point (AP) is an entity that provides access to thedistribution system (DS) via wireless medium for the station associatedtherewith. In the infrastructure BSS, communication among non-APstations is, in principle, performed via the AP, but when a direct linkis configured, direct communication is enabled even among the non-APstations. Meanwhile, in the present invention, the AP is used as aconcept including a personal BSS coordination point (PCP) and mayinclude concepts including a centralized controller, a base station(BS), a node-B, a base transceiver system (BTS), and a site controllerin a broad sense. In the present invention, an AP may also be referredto as a base wireless communication terminal. The base wirelesscommunication terminal may be used as a term which includes an AP, abase station, an eNB (i.e. eNodeB) and a transmission point (TP) in abroad sense. In addition, the base wireless communication terminal mayinclude various types of wireless communication terminals that allocatemedium resources and perform scheduling in communication with aplurality of wireless communication terminals.

A plurality of infrastructure BSSs may be connected with each otherthrough the distribution system (DS). In this case, a plurality of BSSsconnected through the distribution system is referred to as an extendedservice set (ESS).

FIG. 2 illustrates an independent BSS which is a wireless LAN systemaccording to another embodiment of the present invention. In theembodiment of FIG. 2 , duplicative description of parts, which are thesame as or correspond to the embodiment of FIG. 1 , will be omitted.

Since a BSS3 illustrated in FIG. 2 is the independent BSS and does notinclude the AP, all stations STA6 and STA7 are not connected with theAP. The independent BSS is not permitted to access the distributionsystem and forms a self-contained network. In the independent BSS, therespective stations STA6 and STA7 may be directly connected with eachother.

FIG. 3 is a block diagram illustrating a configuration of a station 100according to an embodiment of the present invention. As illustrated inFIG. 3 , the station 100 according to the embodiment of the presentinvention may include a processor 110, a transceiver 120, a userinterface unit 140, a display unit 150, and a memory 160.

First, the transceiver 120 transmits and receives a wireless signal suchas a wireless LAN packet, or the like and may be embedded in the station100 or provided as an exterior. According to the embodiment, thetransceiver 120 may include at least one transmit/receive module usingdifferent frequency bands. For example, the transceiver 120 may includetransmit/receive modules having different frequency bands such as 2.4GHz, 5 GHz, and 60 GHz. According to an embodiment, the station 100 mayinclude a transmit/receive module using a frequency band of 6 GHz ormore and a transmit/receive module using a frequency band of 6 GHz orless. The respective transmit/receive modules may perform wirelesscommunication with the AP or an external station according to a wirelessLAN standard of a frequency band supported by the correspondingtransmit/receive module. The transceiver 120 may operate only onetransmit/receive module at a time or simultaneously operate multipletransmit/receive modules together according to the performance andrequirements of the station 100. When the station 100 includes aplurality of transmit/receive modules, each transmit/receive module maybe implemented by independent elements or a plurality of modules may beintegrated into one chip. In an embodiment of the present invention, thetransceiver 120 may represent a radio frequency (RF) transceiver modulefor processing an RF signal.

Next, the user interface unit 140 includes various types of input/outputmeans provided in the station 100. That is, the user interface unit 140may receive a user input by using various input means and the processor110 may control the station 100 based on the received user input.Further, the user interface unit 140 may perform output based on acommand of the processor 110 by using various output means.

Next, the display unit 150 outputs an image on a display screen. Thedisplay unit 150 may output various display objects such as contentsexecuted by the processor 110 or a user interface based on a controlcommand of the processor 110, and the like. Further, the memory 160stores a control program used in the station 100 and various resultingdata. The control program may include an access program required for thestation 100 to access the AP or the external station.

The processor 110 of the present invention may execute various commandsor programs and process data in the station 100. Further, the processor110 may control the respective units of the station 100 and control datatransmission/reception among the units. According to the embodiment ofthe present invention, the processor 110 may execute the program foraccessing the AP stored in the memory 160 and receive a communicationconfiguration message transmitted by the AP. Further, the processor 110may read information on a priority condition of the station 100 includedin the communication configuration message and request the access to theAP based on the information on the priority condition of the station100. The processor 110 of the present invention may represent a maincontrol unit of the station 100 and according to the embodiment, theprocessor 110 may represent a control unit for individually controllingsome component of the station 100, for example, the transceiver 120, andthe like. That is, the processor 110 may be a modem or amodulator/demodulator for modulating and demodulating wireless signalstransmitted to and received from the transceiver 120. The processor 110controls various operations of wireless signal transmission/reception ofthe station 100 according to the embodiment of the present invention. Adetailed embodiment thereof will be described below.

The station 100 illustrated in FIG. 3 is a block diagram according to anembodiment of the present invention, where separate blocks areillustrated as logically distinguished elements of the device.Accordingly, the elements of the device may be mounted in a single chipor multiple chips depending on design of the device. For example, theprocessor 110 and the transceiver 120 may be implemented while beingintegrated into a single chip or implemented as a separate chip.Further, in the embodiment of the present invention, some components ofthe station 100, for example, the user interface unit 140 and thedisplay unit 150 may be optionally provided in the station 100.

FIG. 4 is a block diagram illustrating a configuration of an AP 200according to an embodiment of the present invention. As illustrated inFIG. 4 , the AP 200 according to the embodiment of the present inventionmay include a processor 210, a transceiver 220, and a memory 260. InFIG. 4 , among the components of the AP 200, duplicative description ofparts which are the same as or correspond to the components of thestation 100 of FIG. 2 will be omitted.

Referring to FIG. 4 , the AP 200 according to the present inventionincludes the transceiver 220 for operating the BSS in at least onefrequency band. As described in the embodiment of FIG. 3 , thetransceiver 220 of the AP 200 may also include a plurality oftransmit/receive modules using different frequency bands. That is, theAP 200 according to the embodiment of the present invention may includetwo or more transmit/receive modules among different frequency bands,for example, 2.4 GHz, 5 GHz, and 60 GHz together. Preferably, the AP 200may include a transmit/receive module using a frequency band of 6 GHz ormore and a transmit/receive module using a frequency band of 6 GHz orless. The respective transmit/receive modules may perform wirelesscommunication with the station according to a wireless LAN standard of afrequency band supported by the corresponding transmit/receive module.The transceiver 220 may operate only one transmit/receive module at atime or simultaneously operate multiple transmit/receive modulestogether according to the performance and requirements of the AP 200. Inan embodiment of the present invention, the transceiver 220 mayrepresent a radio frequency (RF) transceiver module for processing an RFsignal.

Next, the memory 260 stores a control program used in the AP 200 andvarious resulting data. The control program may include an accessprogram for managing the access of the station. Further, the processor210 may control the respective units of the AP 200 and control datatransmission/reception among the units. According to the embodiment ofthe present invention, the processor 210 may execute the program foraccessing the station stored in the memory 260 and transmitcommunication configuration messages for one or more stations. In thiscase, the communication configuration messages may include informationabout access priority conditions of the respective stations. Further,the processor 210 performs an access configuration according to anaccess request of the station. According to an embodiment, the processor210 may be a modem or a modulator/demodulator for modulating anddemodulating wireless signals transmitted to and received from thetransceiver 220. The processor 210 controls various operations such aswireless signal transmission/reception of the AP 200 according to theembodiment of the present invention. A detailed embodiment thereof willbe described below.

FIG. 5 is a diagram schematically illustrating a process in which a STAsets a link with an AP.

Referring to FIG. 5 , the link between the STA 100 and the AP 200 is setthrough three steps of scanning, authentication, and association in abroad way. First, the scanning step is a step in which the STA 100obtains access information of BSS operated by the AP 200. A method forperforming the scanning includes a passive scanning method in which theAP 200 obtains information by using a beacon message (S101) which isperiodically transmitted and an active scanning method in which the STA100 transmits a probe request to the AP (S103) and obtains accessinformation by receiving a probe response from the AP (S105).

The STA 100 that successfully receives wireless access information inthe scanning step performs the authentication step by transmitting anauthentication request (S107 a) and receiving an authentication responsefrom the AP 200 (S107 b). After the authentication step is performed,the STA 100 performs the association step by transmitting an associationrequest (S109 a) and receiving an association response from the AP 200(S109 b). In this specification, an association basically means awireless association, but the present invention is not limited thereto,and the association may include both the wireless association and awired association in a broad sense.

Meanwhile, an 802.1X based authentication step (S111) and an IP addressobtaining step (S113) through DHCP may be additionally performed. InFIG. 5 , the authentication server 300 is a server that processes 802.1Xbased authentication with the STA 100 and may be present in physicalassociation with the AP 200 or present as a separate server.

FIG. 6 is a diagram illustrating a carrier sense multiple access(CSMA)/collision avoidance (CA) method used in wireless LANcommunication.

A terminal that performs a wireless LAN communication checks whether achannel is busy by performing carrier sensing before transmitting data.When a wireless signal having a predetermined strength or more issensed, it is determined that the corresponding channel is busy and theterminal delays the access to the corresponding channel. Such a processis referred to as clear channel assessment (CCA) and a level to decidewhether the corresponding signal is sensed is referred to as a CCAthreshold. When a wireless signal having the CCA threshold or more,which is received by the terminal, indicates the corresponding terminalas a receiver, the terminal processes the received wireless signal.Meanwhile, when a wireless signal is not sensed in the correspondingchannel or a wireless signal having a strength smaller than the CCAthreshold is sensed, it is determined that the channel is idle.

When it is determined that the channel is idle, each terminal havingdata to be transmitted performs a backoff procedure after an inter framespace (IFS) time depending on a situation of each terminal, forinstance, an arbitration IFS (AIFS), a PCF IFS (PIFS), or the likeelapses. According to the embodiment, the AIFS may be used as acomponent which substitutes for the existing DCF IFS (DIFS). Eachterminal stands by while decreasing slot time(s) as long as a randomnumber determined by the corresponding terminal during an interval of anidle state of the channel and a terminal that completely exhausts theslot time(s) attempts to access the corresponding channel. As such, aninterval in which each terminal performs the backoff procedure isreferred to as a contention window interval.

When a specific terminal successfully accesses the channel, thecorresponding terminal may transmit data through the channel. However,when the terminal which attempts the access collides with anotherterminal, the terminals which collide with each other are assigned withnew random numbers, respectively to perform the backoff procedure again.According to an embodiment, a random number newly assigned to eachterminal may be decided within a range (2*CW) which is twice larger thana range (a contention window, CW) of a random number which thecorresponding terminal is previously assigned. Meanwhile, each terminalattempts the access by performing the backoff procedure again in a nextcontention window interval and in this case, each terminal performs thebackoff procedure from slot time(s) which remained in the previouscontention window interval. By such a method, the respective terminalsthat perform the wireless LAN communication may avoid a mutual collisionfor a specific channel.

FIG. 7 is a diagram illustrating a method for performing a distributedcoordination function using a request to send (RTS) frame and a clear tosend (CTS) frame.

The AP and STAs in the BSS contend in order to obtain an authority fortransmitting data. When data transmission at the previous step iscompleted, each terminal having data to be transmitted performs abackoff procedure while decreasing a backoff counter (alternatively, abackoff timer) of a random number assigned to each terminal after anAFIS time. A transmitting terminal in which the backoff counter expirestransmits the request to send (RTS) frame to notify that correspondingterminal has data to transmit. According to an exemplary embodiment ofFIG. 7 , STA1 which holds a lead in contention with minimum backoff maytransmit the RTS frame after the backoff counter expires. The RTS frameincludes information on a receiver address, a transmitter address, andduration. A receiving terminal (i.e., the AP in FIG. 7 ) that receivesthe RTS frame transmits the clear to send (CTS) frame after waiting fora short IFS (SIFS) time to notify that the data transmission isavailable to the transmitting terminal STA1. The CTS frame includes theinformation on a receiver address and duration. In this case, thereceiver address of the CTS frame may be set identically to atransmitter address of the RTS frame corresponding thereto, that is, anaddress of the transmitting terminal STA1.

The transmitting terminal STA1 that receives the CTS frame transmits thedata after a SIFS time. When the data transmission is completed, thereceiving terminal AP transmits an acknowledgment (ACK) frame after aSIFS time to notify that the data transmission is completed. When thetransmitting terminal receives the ACK frame within a predeterminedtime, the transmitting terminal regards that the data transmission issuccessful. However, when the transmitting terminal does not receive theACK frame within the predetermined time, the transmitting terminalregards that the data transmission is failed. Meanwhile, adjacentterminals that receive at least one of the RTS frame and the CTS framein the course of the transmission procedure set a network allocationvector (NAV) and do not perform data transmission until the set NAV isterminated. In this case, the NAV of each terminal may be set based on aduration field of the received RTS frame or CTS frame.

In the course of the aforementioned data transmission procedure, whenthe RTS frame or CTS frame of the terminals is not normally transferredto a target terminal (i.e., a terminal of the receiver address) due to asituation such as interference or a collision, a subsequent process issuspended. The transmitting terminal STA1 that transmitted the RTS frameregards that the data transmission is unavailable and participates in anext contention by being assigned with a new random number. In thiscase, the newly assigned random number may be determined within a range(2*CW) twice larger than a previous predetermined random number range (acontention window, CW).

<Multi-User Transmission>

When using orthogonal frequency division multiple access (OFDMA) ormulti-input multi-output (MIMO), one wireless communication terminal cansimultaneously transmit data to a plurality of wireless communicationterminals. Further, one wireless communication terminal cansimultaneously receive data from a plurality of wireless communicationterminals. For example, a downlink multi-user (DL-MU) transmission inwhich an AP simultaneously transmits data to a plurality of STAs, and anuplink multi-user (UL-MU) transmission in which a plurality of STAssimultaneously transmit data to the AP may be performed.

In order to perform the UL-MU transmission, the channel to be used andthe transmission start time of each STA that performs uplinktransmission should be adjusted. In order to efficiently schedule theUL-MU transmission, state information of each STA needs to betransmitted to the AP. According to an embodiment of the presentinvention, information for scheduling of a UL-MU transmission may beindicated through a predetermined field of a preamble of a packet and/ora predetermined field of a MAC header. For example, a STA may indicateinformation for UL-MU transmission scheduling through a predeterminedfield of a preamble or a MAC header of an uplink transmission packet,and may transmit the information to an AP. In this case, the informationfor UL-MU transmission scheduling includes at least one of buffer statusinformation of each STA, channel state information measured by each STA.The buffer status information of the STA may indicate at least one ofwhether the STA has uplink data to be transmitted, the access category(AC) of the uplink data and the size (or the transmission time) of theuplink data.

According to an embodiment of the present invention, the UL-MUtransmission process may be managed by the AP. The UL-MU transmissionmay be performed in response to a trigger frame transmitted by the AP.The STAs simultaneously transmit uplink data a predetermined IFS timeafter receiving the trigger frame. The trigger frame indicates the datatransmission time point of the uplink STAs and may inform the channel(or subchannel) information allocated to the uplink STAs. When the APtransmits the trigger frame, a plurality of STAs transmit uplink datathrough the respective allocated subcarriers at a time point designatedby the trigger frame. After the uplink data transmission is completed,the AP transmits an ACK to the STAs that have successfully transmittedthe uplink data. In this case, the AP may transmit a predeterminedmulti-STA block ACK (M-BA) as an ACK for a plurality of STAs.

In the non-legacy wireless LAN system, a specific number, for example,26, 52, or 106 tones may be used as a resource unit (RU) for asubchannel-based access in a channel of 20 MHz band. Accordingly, thetrigger frame may indicate identification information of each STAparticipating in the UL-MU transmission and information of the allocatedresource unit. The identification information of the STA includes atleast one of an association ID (AID), a partial AID, and a MAC addressof the STA. Further, the information of the resource unit includes thesize and placement information of the resource unit.

On the other hand, in the non-legacy wireless LAN system, a UL-MUtransmission may be performed based on a contention of a plurality ofSTAs for a particular resource unit. For example, if an AID field valuefor a particular resource unit is set to a specific value (e.g., 0) thatis not assigned to STAs, a plurality of STAs may attempt random access(RA) for the corresponding resource unit.

<Multi-Channel Downlink/Uplink Transmission>

In the wireless LAN system, terminals of each BSS set a specific channelas a primary channel to perform communication. The primary channel is achannel used by non-AP STAs to associate with an AP. The transmissionbandwidth of data can be extended from a basic 20 MHz to 40 MHz, 80 MHz,160 MHz and the like. On the other hand, a secondary channel may beaggregated with the primary channel to form a channel having a bandwidthtwice or more.

The terminals of the BSS perform a CCA on each channel to check whetherthe corresponding channel is busy, and perform bandwidth extension basedon the channel determined to be idle. That is, by assuming 20 MHz as abasic bandwidth, the terminal may extend the transmission bandwidth to40 MHz, 80 MHz, 160 MHz, and the like, according to whether the channelsadjacent to the primary channel are idle. If the transmission bandwidthis extended to non-contiguous channels, the transmission bandwidth canbe extended to various channel configurations such as 20+20 MHz, 20+40MHz, 40+20 MHz, 60 MHz, and the like. The terminal may transmit downlinkdata or uplink data using the extended multi-channel transmissionbandwidth.

Hereinafter, channel access methods for multi-channel downlinktransmission and uplink transmission will be described with reference toFIGS. 8 to 11 . In each embodiment, CH1, CH2, CH3 and CH4 representchannels in units of 20 MHz, respectively. However, the bandwidth ofeach channel may be changed according to the communication method towhich the present invention is applied. In each embodiment, CH1 denotesa primary channel, and CH2 to CH4 denote secondary channels. In eachembodiment shown in FIGS. 8 to 11 , duplicative description of partswhich are the same as or corresponding to the previous embodiment willbe omitted.

FIG. 8 illustrates a channel access method for a multi-channeldownlink/uplink transmission according to an embodiment of the presentinvention. In the embodiment of FIG. 8 , the terminal transmits a PLCPprotocol data unit (PPDU) using an 80 MHz bandwidth.

In each embodiment, after the completion of a transmission of first data(e.g., PPDU-1), the AP may perform a post backoff procedure for themulti-channel before the generation of second data (e.g., PPDU-2) to betransmitted next. In the case of a downlink transmission, the first dataand the second data represent downlink data, and in the case of anuplink transmission, the first data and the second data represent uplinkdata. According to a further embodiment of the present invention, thefirst data may represent downlink data transmitted by an AP, and thesecond data may represent uplink data transmitted by a STA.

In the embodiment of the present invention, the post backoff procedureindicates a backoff procedure that is performed in advance for the nextchannel access after the data transmission of the terminal is completed.If the channel is idle for an AIFS time after data transmission iscompleted, the terminal performs a new backoff procedure. In this case,the terminal obtains a new backoff counter within a contention windowand performs a new backoff procedure based on the obtained backoffcounter. According to an embodiment, the post backoff procedure mayindicate a backoff procedure performed while data to be transmitted bythe corresponding terminal is not present yet.

When the back off counter of the post back off procedure expires, theterminal is in a transmission standby state. If data to be transmittedoccurs in the transmission standby state, the terminal may attempt totransmit the data after a CCA for a predetermined time withoutperforming a separate backoff procedure. More specifically, the terminalperforms a CCA respectively for an AIFS time in the primary channel andfor a predetermined time in the secondary channel, and immediatelyperforms a transmission of the corresponding data when all the channelsin which the CCA is performed are idle.

First, FIG. 8(a) shows a process of transmitting a PPDU using an 80 MHzbandwidth. After the transmission of the first data (e.g., PPDU-1) inthe 80 MHz band is completed, the terminal performs a CCA of the primarychannel CH1 for an AIFS time. If the channel is idle for an AIFS time,the terminal performs a post back off procedure in the primary channel.That is, the terminal performs a CCA by decrementing a randomly selectedbackoff counter value within the contention window by one. The terminalperforms a CCA on the secondary channels through which data is to betransmitted for a predetermined yIFS time before the backoff counter ofthe backoff procedure expires. In the embodiment of the presentinvention, yIFS denotes ‘PIFS+n*slot time’ (herein, n is an integergreater than or equal to 1). That is, the terminal may perform the CCAon the secondary channels for a longer time than the PIFS. According toan embodiment, when at least one secondary channel is busy during theyIFS time, the post backoff procedure for the multi-channel may bedetermined as failed.

In the example of FIG. 8(a), the terminal assigns a backoff counter 4 inthe first post backoff procedure 42. The CH3 is busy during the yIFStime before the backoff counter of the first post backoff procedure 42expires and the backoff procedure 42 is determined to be failed. Theterminal performs the second post backoff procedure 44 based on a newbackoff counter after an AIFS time. According to an embodiment, theterminal may double the size of the contention window and obtain a newbackoff counter within the increased contention window. In the secondpost backoff procedure 44, the terminal assigns a backoff counter 5.However, during the second post backoff procedure 44, CH1 becomes a busystate and the backoff procedure 44 is suspended. Thereafter, when theCH1 becomes idle again, the terminal resumes the second post backoffprocedure 44 after an AIFS time. All the secondary channels are idle forthe yIFS time before the backoff counter of the second post backoffprocedure 44 expires, and the corresponding backoff procedure 44 issuccessfully terminated. In this case, the contention window is reset toan initial value.

When the second post backoff procedure 44 is successfully terminated,the terminal is in the data transmission standby state. If the seconddata (e.g., PPDU-2) to be transmitted occurs in the transmission standbystate, the terminal may immediately attempt to transmit thecorresponding data (e.g., PPDU-2) without performing a separate backoffprocedure. The terminal performs a CCA for a predetermined time for thedata transmission. In this case, the terminal performs the CCA for anAIFS time in the primary channel CH1 and for a yIFS time in thesecondary channels CH2, CH3 and CH4. If all the channels in which theCCA is performed are idle, the terminal performs a transmission of thesecond data (e.g., PPDU-2). However, if at least one channel among thechannels in which the CCA is performed is busy, the terminal performs anew backoff procedure 46. In the embodiment of FIG. 8(a), the terminaltransmits the second data (e.g., PPDU-2) after the new backoff procedure46 is terminated.

According to the embodiment of the present invention, the terminal mayperform the CCA for a predetermined yIFS time using various methods.First, the terminal may perform the CCA using an energy detectiontechnique. That is, the terminal checks whether a level of any receivedsignal is higher than a preset ED threshold. Next, the terminal mayperform the CCA using a correlation detection technique. According to anexemplary embodiment, the terminal may detect a pattern in which acyclic prefix (CP) of an 64 FFT-based orthogonal frequency domainmultiplexing (OFDM) symbol is periodically repeated, and may checkwhether the level of the corresponding signal is higher than a preset CDthreshold (e.g., CD threshold-1). In this case, the length of the CP is0.4 us or 0.8 us. According to another embodiment, the terminal maydetect a pattern in which a CP of a 256 FFT-based OFDM symbol isperiodically repeated, and may check whether the level of thecorresponding signal is higher than a preset CD threshold (e.g., CDthreshold-2). In this case, the length of the CP is 0.8 us, 1.6 us, or3.2 us. In this case, the terminal can quickly detect the patternthrough a long OFDM symbol length and a CP length different from that ofthe conventional 64 FFT. According to yet another embodiment, theterminal may check whether the level of an OFDM signal detected atintervals of 78.125 kHz according to the frequency characteristic of 256FFT-based OFDM is higher than a predetermined CD threshold (e.g., CDthreshold-3).

Next, FIG. 8(b) shows another embodiment of transmitting a PPDU using an80 MHz bandwidth. The terminal may perform the post backoff procedure 42similar to the embodiment of FIG. 8(a). However, the terminal may omitthe CCA of the secondary channels CH2, CH3 and CH4 for the predeterminedyIFS time in the post backoff procedure 42. That is, the terminalperforms the CCA of the secondary channels CH2, CH3 and CH4 for the yIFStime only in the backoff procedure that proceeds with having data to betransmitted.

Referring to FIG. 8(b), when the backoff counter of the post backoffprocedure 42 expires, the terminal is in a transmission standby state.In the post backoff procedure 42, the terminal performs a CCA only forthe primary channel CH1 and does not perform a CCA for the secondarychannels CH2, CH3 and CH4. The terminal waits until data to betransmitted occurs. When the second data (e.g., PPDU-2) to betransmitted occurs, the terminal performs a CCA for a predetermined timein the primary channel CH1 and the secondary channels CH2, CH3 and CH4for a data transmission. In this case, the terminal performs the CCA foran AIFS time in the primary channel CH1 and for a yIFS time in thesecondary channels CH2, CH3 and CH4. If all the channels in which theCCA is performed are idle, the terminal performs a transmission of thesecond data (e.g., PPDU-2). However, since CH2 is in a busy state, theterminal performs a new backoff procedure 46. The terminal transmits thesecond data (e.g., PPDU-2) after the new backoff procedure 46 isterminated.

Next, FIG. 8(c) shows yet another embodiment of transmitting a PPDUusing an 80 MHz bandwidth. According to yet another embodiment of thepresent invention, the terminal may perform a CCA for an AIFS time inall the transmission channels CH1, CH2, CH3 and CH4 after the datatransmission is completed, and may perform the post backoff procedure 42when all the channels in which the CCA is performed are idle. Referringto FIG. 8(c), after the transmission of the first data (e.g., PPDU-1) iscompleted, the terminal performs a CCA for the AIFS time in the primarychannel CH1 and the secondary channels CH2, CH3 and CH4. However, sinceCH3 is in a busy state, the terminal postpones the post backoffprocedure 42. After CH3 becomes idle, the terminal performs the CCA foran AIFS time in all the transmission channels CH1, CH2, CH3 and CH4again. However, since CH1 is in a busy state, the terminal postpones thepost backoff procedure 42 again. After CH1 becomes idle, the terminalperforms the CCA for an AIFS time in all the transmission channels CH1,CH2, CH3 and CH4 again. Since all the channels in which the CCA isperformed are idle, the terminal performs the post backoff procedure 42.

According to the embodiment of FIG. 8(c), the post backoff procedure 42is performed in all the transmission channels CH1, CH2, CH3 and CH4.That is, the terminal performs the post backoff procedure 42 in theprimary channel CH1 and the secondary channels CH2, CH3 and CH4 usingthe obtained backoff counter. If at least one channel becomes busyduring the post back off procedure 42, the post backoff procedure 42 issuspended. When the post backoff procedure 42 is terminated, theterminal is in a transmission standby state.

When the second data (e.g., PPDU-2) to be transmitted occurs, theterminal performs a CCA for a predetermined time in the primary channelCH1 and the secondary channels CH2, CH3 and CH4 for a data transmission.In this case, the terminal performs the CCA for an AIFS time in theprimary channel CH1 and the secondary channels CH2, CH3 and CH4. If allthe channels in which the CCA is performed are idle, the terminalperforms a transmission of the second data (e.g., PPDU-2). However,since CH2 is in a busy state, the terminal performs a new backoffprocedure 46. The terminal transmits the second data (e.g., PPDU-2)after the new backoff procedure 46 is terminated. According to anembodiment, the terminal may perform the new backoff procedure 46 in allthe transmission channels CH1, CH2, CH3 and CH4.

According to still another embodiment, the first data (e.g., PPDU-1) maybe downlink data and the second data (e.g., PPDU-2) may be uplink data.When the AP signals a non-AP of a success of the post backoff procedurefollowing the downlink data transmission as shown in FIG. 8 , the non-APSTA may transmit uplink data after briefly performing a backoffprocedure of the uplink data transmission using the correspondinginformation as shown in FIG. 8 .

FIG. 9 illustrates a channel access method for a multi-channel uplinkmulti-user transmission according to an embodiment of the presentinvention. In the embodiment of FIG. 9 , a UL-MU transmission process isperformed using an 80 MHz bandwidth including CH1, CH2, CH3 and CH4. Inthis case, STA1, STA2, STA3 and STA4 are allocated to CH1, CH2, CH3 andCH4, respectively, to transmit uplink data.

First, FIG. 9(a) shows a multi-channel UL-MU transmission processaccording to an embodiment of the present invention. The AP performs abackoff procedure for the initiation of a UL-MU transmission process310. When the backoff counter of the backoff procedure expires, the APtransmits a trigger frame (e.g., Trigger 1). In the embodiment of FIG.9(a), the trigger frame transmitted by the AP contains resourceallocation information for STA1 to STA4. STA1 to STA4 which havereceived the trigger frame transmit uplink multi-user data (e.g., ULData 1) through respective allocated resource units after a xIFS time.According to an embodiment, the xIFS is a SIFS. The AP receives uplinkmulti-user data transmitted by the STA1 to STA4 and transmits amulti-STA block ACK (M-STA BA) after a SIFS time in response thereto.

According to the embodiment of the present invention, after the UL-MUtransmission process 310 is completed, the AP performs a post backoffprocedure 42 for multi-channels. As described in the previousembodiment, the post backoff procedure 42 for the multi-channel includesa backoff procedure in the primary channel CH1 and a CCA for apredetermined yIFS time in the secondary channels CH2, CH3 and CH4. Whenthe post backoff procedure 42 for the multi-channel is successfullyterminated, the AP is in a transmission standby state. The AP receivesbuffer status report from STAs and starts the next UL-MU transmissionprocess 320 based on the received buffer status report. In this case,the AP may start the UL-MU transmission process 320 after performing aCCA for a predetermined time without performing a separate backoffprocedure. That is, the AP performs the CCA for the AIFS time in theprimary channel CH1 and for the yIFS time in the secondary channels CH2,CH3 and CH4, transmits a trigger frame (e.g., Trigger 2) when all thechannels in which the CCA is performed are idle.

FIG. 9(b) shows a post backoff procedure when the UL-MU transmissionprocess has failed. The AP transmits a trigger frame (e.g., Trigger 1)for the initiation of a UL-MU transmission process 312. However, if nouplink data corresponding to the trigger frame is received at all, theAP determines that the UL-MU transmission process 312 has failed. Inthis case, the AP may perform a multi-channel post backoff procedure 42for the next transmission. Since the previous UL-MU transmission process312 has failed, the AP doubles the size of the contention window andobtains a new backoff counter within the increased contention window.Thereafter, the AP performs the post backoff procedure 42 using the newbackoff counter. According to an embodiment, the post backoff procedure42 may be performed after a time when it is determined that the UL-MUtransmission process 312 designated by the trigger frame has not beenperformed.

On the other hand, when at least one uplink data is received, the AP maydetermine that the UL-MU transmission process is successful. In thiscase, the AP may reset the contention window used in the previousbackoff procedure to an initial value, obtain a new backoff counterbased thereon, and perform a post backoff procedure for the nexttransmission using the new backoff counter.

FIG. 10 illustrates a channel access method for a multi-channel uplinkmulti-user transmission according to another embodiment of the presentinvention. FIG. 10(a) shows a contiguous multi-channel transmissionmethod, and FIG. 10(b) shows a non-contiguous multi-channel transmissionmethod.

First, referring to FIG. 10(a), after the UL-MU transmission process 310is completed, the AP performs the post backoff procedure 42 for themulti-channel. The AP performs the CCA procedure for the yIFS time forthe secondary channels CH2, CH3 and CH4 in the post backoff procedure42. When the backoff counter of the post backoff procedure 42 expires,the AP may immediately transmit a trigger frame (e.g., Trigger2) for theinitiation of the next UL-MU transmission process 322. Since CH3 is busyin the CCA procedure, the AP transmits the trigger frame (e.g.,Trigger2) through a 40 MHz channel including CH1 and CH2 based on acontiguous channel extension principle. The trigger frame (e.g.,Trigger2) contains resource allocation information for the STA1 and theSTA2 allocated to CH1 and CH2, respectively. In the UL-MU transmissionprocess 322, STA1 and STA2 transmit uplink multi-user data using CH1 andCH2.

Next, referring to FIG. 10(b), UL-MU transmission processes 314 and 324may be performed based on a non-contiguous channel extension principle.The AP performs the backoff procedure and the CCA procedure for thesecondary channels CH2, CH3 and CH4 for the initiation of the UL-MUtransmission process 312. In this case, since CH3 is busy, the APtransmits a trigger frame (e.g., Trigger1) through a 40 MHz channelincluding CH1 and CH2 and a 20 MHz channel of CH4. The trigger frame(e.g., Trigger1) contains resource allocation information for STA1, STA2and STA4 allocated CH1, CH2 and CH4, respectively. In the UL-MUtransmission process 314, STA1, STA2 and STA4 transmit uplink multi-userdata.

When the UL-MU transmission process 312 is completed, the AP performsthe post backoff procedure 42 for the multi-channel. The AP performs theCCA procedure for the yIFS time for the secondary channels CH2, CH3 andCH4. In the embodiment of FIG. 10(b), CH2 is busy. The AP transmits atrigger frame (e.g., Trigger2) through a 20 MHz channel of CH1 and a 40MHz channel including CH3 and CH4 based on the non-contiguous channelextension principle. The trigger frame (e.g., Trigger2) includesresource allocation information for STA1, STA3 and STA4 allocated CH1,CH3 and CH4, respectively. In the UL-MU transmission process 324, STA1,STA3 and STA4 transmit uplink multi-user data.

FIG. 11 illustrates a multi-channel uplink multi-user transmissionmethod according to yet another embodiment of the present invention. Inthe embodiment of FIG. 11 , a UL-MU transmission process is performedusing a 40 MHz bandwidth including CH1 and CH2. In this case, it isassumed that STA0, STA1 and STA2 are allocated to CH1, and STA3, STA4and STA5 are allocated to CH2.

First, referring to FIG. 11(a), the AP transmits a trigger frame throughCH1 and CH2. The trigger frame transmitted by the AP indicates thatSTA0, STA1 and STA2 to transmit uplink multi-user data through CH1, andthat STA3, STA4 and STA5 to transmit uplink multi-user data through CH2.Each STA receiving the trigger frame transmits uplink multi-user datathrough a channel assigned to each terminal after performing a CCA forxIFS time. According to an embodiment, the xIFS is a SIFS. The APreceives the uplink multi-user data transmitted by the STAID to STA5 andtransmits an M-STA BA after a SIFS time in response thereto.

The embodiment of FIG. 11(b) shows a situation in which CH2 is busy whenthe trigger frame is transmitted by the AP. STA3, STA4 and STA5 cannottransmit uplink data corresponding to the trigger frame since the CH2allocated to the terminals is busy. The AP receives uplink multi-userdata transmitted by STA0, STA1 and STA2 through CH1 and transmits anM-STA BA in response thereto. The M-STA BA transmitted by the APcontains ACK information for STA0, STA1 and STA2. According to theembodiment of the present invention, the AP may transmit the M-STA BAnot only through CH1 but also through CH2 in which UL-MU transmission isnot performed. As described above, the AP may transmit the M-STA BAthrough a channel in which the UL-MU transmission has failed, therebysecuring resources for the next multi-channel transmission process.

The embodiment of FIG. 11(c) shows a situation in which CH2 is busy onlyfor STA3 when the trigger frame is transmitted by the AP. The STA3cannot transmit uplink data corresponding to the trigger frame since theCH2 allocated to the terminal is busy. The AP receives uplink multi-userdata transmitted by STA0, STA1 and STA2 through CH1 and uplinkmulti-user data transmitted by STA4 and STA5 through CH2. The APtransmits an M-STA BA in response to the received uplink multi-userdata. The M-STA BA transmitted by the AP contains ACK information forSTA0, STA1, STA2, STA4 and STA5.

Meanwhile, according to a further embodiment of the present invention,the resource allocation information of the trigger frame may betransmitted in duplicate in units of 20 MHz channels. That is, in theabove embodiments, the allocation information of CH1 for STA0 to STA2and allocation information of CH2 for STA3 to STA5 are transmittedtogether through CH1 and CH2. Accordingly, the STAs can obtain theresource allocation information of the entire bandwidth in which theUL-MU transmission is performed from a trigger frame received via atleast one channel. In the embodiments of FIGS. 11(b) and 11(c), even ifa channel allocated to a specific STA is busy at the time when thetrigger frame is transmitted by the AP, the corresponding STA maytransmit uplink multi-user data based on the resource allocationinformation of the trigger frame received via the other channel.

FIG. 12 illustrates a periodic uplink multi-user data transmissionmethod according to a further embodiment of the present invention.According to a further embodiment of the present invention, the terminalmay transmit data at a predetermined transmission timing (e.g., UL Time1, UL Time 2, UL Time 3). The predetermined transmission timing may be asynchronized timing. As in other embodiments of the present invention,in the embodiment of FIG. 12 , the AP and the STA may be replaced byenhanced Node B (eNB) and user equipment (UE), respectively.

According to the embodiment of FIG. 12 , the AP transmits an uplinkgrant message 50 to STA1 and STA2. In the embodiment of the presentinvention, the uplink grant message 50 includes the above-describedtrigger frame, and may further include various types of messagesindicating uplink transmission of a terminal. In the uplink grantmessage 50, transmission timing information, transmission periodinformation, and the like, for transmitting uplink data may be indicatedfor each STA.

The STAs may perform a backoff procedure in advance before thedesignated transmission timing to determine whether to transmit uplinkdata at the corresponding transmission timing. In this case, the backoffprocedure to be performed in advance may be performed in the same orcorresponding manner as the post backoff procedure of theabove-described embodiments. If the backoff counter of the backoffprocedure expires before the transmission timing arrives, the STA is ina transmission standby state. The STA performs a CCA for a predeterminedtime immediately before the transmission timing, and transmits uplinkdata when the channel is idle. According to an embodiment, thepredetermined time may be an AIFS, but the present invention is notlimited thereto. When the transmission of uplink data based on thetransmission timing is completed, the STA may perform the backoffprocedure described above again.

On the other hand, if the backoff procedure is not completed before thetransmission timing arrives, the STA delays the transmission of uplinkdata until the backoff counter of the backoff procedure expires. Thatis, the STA transmits uplink data when the backoff counter of thecorresponding backoff procedure expires. Therefore, the uplink datatransmission start time of a STA in which the backoff procedure is notcompleted may be delayed from the predetermined transmission timing.

Referring to the embodiment of FIG. 12(a), the uplink grant message 50may indicate information of STAs to transmit uplink data at eachtransmission timing. STA1 and STA2, which are indicated to transmit atthe first transmission timing (e.g., UL Time 1), respectively perform abackoff procedure before the first transmission timing (e.g., UL Time 1)arrives. In the embodiment of FIG. 12(a), the backoff procedures of STA1and STA2 are completed before the first transmission timing (e.g., ULTime 1). STA1 and STA2 perform a CCA for an AIFS time before the firsttransmission timing (e.g., UL Time 1), and respectively transmit uplinkdata since the channel is idle. After transmission of the uplink data,STA1 and STA2 respectively perform a backoff procedure based on a newbackoff counter allocated thereto.

At the second transmission timing (e.g., UL Time 2), uplink datatransmission of STA1 is indicated. In the embodiment of FIG. 12(a), thebackoff procedure of the STA1 is completed before the secondtransmission timing (e.g., UL Time 2), and the STA1 performs a CCA foran AIFS time before the second transmission timing (e.g., UL Time 2).Since the channel is idle for an AIFS time, the STA1 transmits uplinkdata. At the third transmission timing (e.g., UL Time 3), uplink datatransmission of STA2 is indicated. STA2 transmits uplink data after aCCA for an AIFS time before the third transmission timing (e.g., UL Time3) since the STA2 has already completed the prior backoff procedure andis in the transmission standby state.

Referring to the embodiment of FIG. 12(b), the channel is busy duringthe backoff procedure of STA1 before the first transmission timing(e.g., UL Time 1). STA1 suspends the backoff procedure and resumes thebackoff procedure after an AIFS time when the channel is idle. However,the backoff procedure of STA1 is not completed before the firsttransmission timing (e.g., UL Time 1) arrives. Therefore, the STA1delays the transmission of uplink data until the backoff counter of thebackoff procedure expires. When the backoff counter of the backoffprocedure expires, the STA1 may transmit the uplink data. According toan exemplary embodiment, the STA1 may determine whether to transmituplink data considering the size of the data to be transmitted and thedelayed time. That is, if the transmission period of the uplink data tobe transmitted exceeds the next transmission timing due to the delayedtime, the transmission of the uplink data may not be performed.

At the second transmission timing (e.g., UL Time 2), uplink datatransmission of STA1 is indicated. In the embodiment of FIG. 12(b), thebackoff procedure of the STA1 is completed before the secondtransmission timing (e.g., UL Time 2). However, the channel is busyduring the CCA interval for an AIFS time and the STA1 fails to transmituplink data. STA1 that fails to transmit uplink data performs a backoffprocedure based on a new backoff counter an AIFS time after the channelbecomes idle. At the third transmission timing (e.g., UL Time 3), uplinkdata transmissions of STA1 and STA2 are indicated. Since the priorbackoff procedures of STA1 and STA2 have already been completed, STA1and STA2 transmit uplink data after a CCA for an AIFS time before thethird transmission timing (e.g., UL Time 3).

FIG. 13 illustrates an uplink multi-user transmission process accordingto the embodiment of the present invention. As described above, theUL-MU transmission may be performed in response to a trigger frametransmitted by the AP. STAs simultaneously transmit uplink data an xIFStime after receiving the trigger frame. According to an embodiment, thexIFS may be a SIFS. When the AP transmits the trigger frame, a pluralityof STAs transmit uplink data through the respective assigned subcarriersat the time designated by the trigger frame. The AP transmits an M-STABA to STAs that have succeeded in the uplink data transmission a SIFStime after the uplink data transmission is completed. The length of theM-STA BA may vary depending on the number of target STAs of the ACKinformation. According to an embodiment, the length of the M-STA BA maybe determined within a maximum BA length (e.g., d_max_BA).

In the UL-MU transmission process, the transmission performance may varydepending on the format of the trigger frame. To determine the format ofthe trigger frame, the following factors need to be considered. First,signaling efficiency should be considered. In other words, it should beconsidered how small packet overhead is possible to transmit the triggerframe. Next, the decoding performance of the trigger frame should beconsidered. That is, it should be able to receive the trigger framereliably even in the interference of other BSS (OBSS) or an outdoorenvironment. Next, the reception of uplink multi-user data of the APshould be protected from hidden nodes adjacent to the AP. In addition,the reception of downlink M-STA BA of each uplink STA should beprotected from hidden nodes adjacent to the corresponding STA. In thiscase, since the length of the M-STA BA is variable, protection should beperformed up to the maximum BA length (e.g., d_max_BA).

According to the embodiment of the present invention, various types oftrigger frames are proposed. According to an embodiment, a trigger frameof null data packet (NDP) format may be used. The NDP is a packet formatthat contains only a PHY header and does not contain a MAC frame.According to another embodiment, a trigger frame of MAC format may beused. In this case, the trigger frame of the MAC format may betransmitted in various PPDU formats of a legacy wireless LAN system(e.g., 802.11a, n, ac) and/or a non-legacy wireless LAN system (e.g.,802.11ax).

Hereinafter, embodiments of the present invention will be describedunder the following assumptions. However, at least some of theassumptions may be changed or omitted according to an embodiment.

First, the performance of the trigger frame can be supplemented byadditional message exchange such as RTS/CTS before the transmission ofthe trigger frame. According to an embodiment of the present invention,the UL-MU transmission process may be controlled through a trigger framewithout an additional RTS/CTS transmission sequence. According toanother embodiment of the present invention, the UL-MU transmissionprocess may be controlled with the aid of an additional RTS/CTStransmission sequence.

Second, legacy STAs may identify the received non-legacy PPDU (i.e., HEPPDU) as an 802.11a PPDU based on the legacy preamble of that PPDU andattempt the decoding process of the MAC frame. However, in the decodingprocess, a frame check sequence (FCS) error of the MAC frame occurs.Therefore, the legacy STAs further waits for an extended IFS (EIFS)interval after the time indicated by an L-SIG of the legacy preamble ofthe corresponding PPDU and performs a backoff procedure to attemptchannel access.

Third, the length of the non-legacy PPDU may be indicated by acombination of a Length field and a Rate field included in the L-SIG orthe repeated L-SIG of the corresponding PPDU.

Fourth, regarding to the non-legacy PPDU, a transmission suitable for anoutdoor environment can be performed by repeating some preamble symbolsor transmitting with an extended length of an OFDM cyclic prefix (CP).

FIG. 14 illustrates an embodiment of a placement situation of terminalsaround a specific BSS. In the embodiment of FIG. 14 , the APcommunicates with STA1 and STA2, and hidden nodes L1, H1, L2 and H2exist on the basis of a specific terminal. Here, L1 and L2 denote legacySTAs, respectively, and H1 and H2 denote non-legacy STAs, respectively.

L1 and H1 are capable of sensing messages of the AP, but cannot receivemessages of STA1 and STA2. Thus, L1 and H1 may interfere with the APwhen the AP receives messages from STA1 and STA2. On the other hand, L2and H2 are capable of sensing messages of STA2, but cannot receivemessages of the AP. Thus, L2 and H2 may interfere with STA2 when STA2receives a message from the AP. Hereinafter, the embodiments of FIGS. 15to 17 will be described on the assumption of the placement of terminalsin FIG. 14 .

FIGS. 15 to 17 illustrate various embodiments of an uplink multi-usertransmission process and operations of hidden nodes according to theprocess. In the embodiment of FIGS. 15 to 17 , the UL-MU transmissionprocess is performed through a 40 MHz band including CH1 and CH2. When atrigger frame for the initiation of the UL-MU transmission process istransmitted, an interference of the OBSS occurs to the CH2. In eachembodiments shown in FIGS. 15 to 17 , duplicative description of partswhich are the same as or corresponding to the previous embodiment willbe omitted.

FIG. 15 illustrates an uplink multi-user transmission process accordingto an embodiment of the present invention and operations of hidden nodesaccording to the process. According to the embodiment of FIG. 15 , atrigger frame 52 in the NDP format is used for the UL-MU transmissionprocess.

The NDP trigger frame 52 includes a legacy preamble (i.e., L-Preamble)and a non-legacy NDP preamble (i.e., HE-NDP Preamble). The legacypreamble includes a legacy short training field (L-STF), a legacy longtraining field (L-LTF) and a legacy signal field (L-SIG). In addition,the non-legacy NDP preamble includes a repeated L-SIG (RL-SIG), a highefficiency signal field A (HE-SIG-A), and a high efficiency signal fieldB (HE-SIG-B).

When the NDP trigger frame 52 is transmitted through a multi-channel,the legacy preamble portion of the NDP trigger frame 52 may betransmitted in duplicate in units of 20 MHz channels. The non-legacy NDPpreamble is transmitted after the legacy preamble. The non-legacy NDPpreamble is transmitted via a 64 FFT-based signal. According to anembodiment, a 256-FFT based high efficiency short training field(HE-STF) and a high efficiency long training field (HE-LTF) may beomitted from the non-legacy NDP preamble.

The NDP trigger frame 52 includes information indicating that thecorresponding frame is a trigger frame. According to an embodiment, theinformation indicating a trigger frame may be represented by apredetermined field of any one of L-SIG of the legacy preamble, andRL-SIG, HE-SIG-A or HE-SIG-B of the non-legacy NDP preamble. Accordingto another embodiment, the information indicating a trigger frame may berepresented by a reserved bit field or an unused subcarrier of thelegacy preamble. According to yet another embodiment, the informationindicating that the corresponding frame is a trigger frame may berepresented through phase rotation of symbols, transmission of anorthogonal sequence, or the like.

The NDP trigger frame 52 may be robustly received in an outdoorenvironment. However, when OBSS interference occurs in CH2 as shown inFIG. 15 , it may be difficult for STAs to receive the NDP trigger frame52. Accordingly, the AP may robustly transmit the HE-SIG-B of the NDPtrigger frame 52. According to an embodiment, the HE-SIG-B may betransmitted in duplicate in units of 20 MHz channels. According toanother embodiment, the HE-SIG-B may be transmitted with a predeterminedrobust modulation and coding scheme (MCS). According to yet anotherembodiment, the HE-SIG-B may be transmitted on a 20 MHz channel basisand only represent resource information of STAs allocated to thecorresponding channel for each 20 MHz channel. According to stillanother embodiment, when the HE-SIG-B comprises a plurality of symbols,the MCS may be set differently for each symbol. According to still yetanother embodiment, a cyclic redundancy check (CRC) code may betransmitted for each 20 MHz channel through which the NDP trigger frame52 is transmitted so that the HE-SIG-B can be analyzed for each channel.

STAs receiving the NDP trigger frame 52 simultaneously transmit anuplink PPDU 60 after an xIFS time. According to an embodiment, the xIFSmay be a SIFS. The uplink multi-user PPDU (UL-MU PPDU) 60 transmitted bya plurality of STAs includes a legacy preamble (i.e., L-Preamble) and anon-legacy preamble (i.e., HE-Preamble). The legacy preamble includesL-STF, L-LTF and L-SIG. In addition, the non-legacy preamble includesRL-SIG, HE-SIG-A, HE-STF and HE-LTF. According to an embodiment,HE-SIG-B indicating information for individual STAs may be omitted fromthe trigger frame-based UL-MU PPDU 60.

The AP transmits an M-STA BA to STAs that have succeeded in the uplinkdata transmission a SIFS time after the transmission of the UL-MU PPDU60 is completed. The length of the M-STA BA may vary depending on thenumber of target STAs of the ACK information. According to anembodiment, the length of the M-STA BA may be determined within amaximum BA length (e.g., d_max_BA).

According to the embodiment of the present invention, the preamble ofthe non-legacy packet transmitted by the AP and the STA may containremaining time information of the current transmission opportunity(TXOP). More specifically, the non-legacy preamble of the NDP triggerframe 52 and the UL-MU PPDU 60 transmitted in the UL-MU transmissionprocess contains information of the remaining TXOP time of the currentTXOP. The remaining TXOP time information may indicate the remainingtime until the completion of the M-STA BA transmission of thecorresponding UL-MU transmission process. As described above, when thelength of M-STA BA is variable, the remaining TXOP time information maybe set based on the maximum BA length (e.g., d_max_BA). When a pluralityof UL-MU transmission processes are performed in the same TXOP as in thefollowing embodiments, the remaining TXOP time information may indicatethe time until the completion of the last UL-MU transmission process.

According to the embodiment of the present invention, the remaining TXOPtime information may be represented by a predetermined TXOP durationfield of the HE-SIG-A of the non-legacy preamble. According to anembodiment, the TXOP duration field of the HE-SIG-A may consist of fewerbits than a TXOP field of a MAC header of the corresponding packet.Therefore, the TXOP duration field of the HE-SIG-A may indicate TXOPtime information in a predetermined unit, for example, an OFDM symbolunit. According to another embodiment of the present invention, theremaining TXOP time information may be represented by a combination of alegacy preamble and a non-legacy preamble. According to an embodiment,the remaining TXOP time information may be represented by a combinationof a Length field and a Rate field of the L-SIG, and a predeterminedfield of the HE-SIG-A. For example, when the remaining TXOP time is setto an integer times of Length field information of the L-SIG, thepredetermined field of the HE-SIG-A may indicate information of theinteger scaling factor.

The UL-MU transmission non-participating terminals that have received atleast one of the NDP trigger frame 52 and the UL-MU PPDU 60 set anetwork allocation vector (NAV) based on the remaining TXOP timeinformation. By inserting the TXOP information into the preamble of thepacket, the terminals may obtain the TXOP information and set the NAVearlier than when the TXOP information is inserted into the conventionalMAC header.

Referring to the embodiment of FIG. 14 , the terminals L1 and H1 receivethe NDP trigger frame 52 transmitted by the AP. In addition, theterminals L2 and H2 receive the UL-MU PPDU 60 transmitted by the STA2.The operations of the neighboring terminals L1, H1, L2 and H2 in theUL-MU transmission process of FIG. 15 are as follows.

First, the non-legacy terminal H1 receiving the NDP trigger frame 52obtains the remaining TXOP time information included in the non-legacypreamble of the corresponding packet, and sets a NAV based thereon.

Meanwhile, the legacy terminal L1 receiving the NDP trigger frame 52estimates a length of the packet based on the Length field of the L-SIGof the corresponding packet. The terminal L1 identifies the NDP triggerframe 52 as an 802.11a packet and performs an FCS check. However, theterminal L1 accesses the channel after an EIFS time due to an error.However, if the Length field of the L-SIG indicates the length of onlythe corresponding packet 52, a collision may occur when the AP receivesthe UL-MU PPDU 60 due to the channel access of the terminal L1.According to the embodiment of the present invention, in order toprevent such collision, the Length field of the L-SIG of the NDP triggerframe 52 may be set based on the TXOP information of the correspondingpacket. That is, the Length field of the L-SIG of the NDP trigger frame52 may be set based on the remaining time until the completion of theM-STA BA transmission of the corresponding UL-MU transmission process.Accordingly, the terminal L1 receiving the NDP trigger frame 52 may seta NAV for the corresponding TXOP duration based on the Length field ofthe L-SIG and delay the transmission.

The non-legacy terminal H2 receiving the UL-MU PPDU 60 obtains theremaining TXOP time information contained in the non-legacy preamble ofthe corresponding packet and sets a NAV based thereon. Meanwhile,according to another embodiment of the present invention, the terminalH2 may set the NAV by additionally using duration field information of aMAC header of the UL-MU PPDU 60. In this case, since the UL-MU PPDU 60is transmitted using MIMO or OFDMA, it may be difficult for neighboringterminals to overhear only the corresponding packet and decode the MACheader. Accordingly, the terminal H2 may decode the MAC header of theUL-MU PPDU 60 by referring to at least one of information extracted fromthe NDP trigger frame 52 and information extracted from the HE-SIG-A ofthe UL-MU PPDU 60. The terminal H2 may set a NAV based on information ofat least one duration field among the decoded multi-user MAC headerinformation.

The legacy terminal L2 receiving the UL-MU PPDU 60 estimates a length ofthe packet based on the Length field of the L-SIG of the correspondingpacket. The terminal L2 identifies the UL-MU PPDU 60 as an 802.11apacket and performs an FCS check. However, the terminal L2 accesses thechannel after an EIFS time due to an error. However, if the Length fieldof the L-SIG indicates the length of only the corresponding packet 60, acollision may occur when the STA2 receives the M-STA BA due to thechannel access of the terminal L2. Therefore, according to theembodiment of the present invention, the Length field of the L-SIG ofthe UL-MU PPDU 60 may be set based on the TXOP information of thecorresponding packet. That is, the Length field of the L-SIG of theUL-MU PPDU 60 may be set based on the remaining time until thecompletion of the M-STA BA transmission of the corresponding UL-MUtransmission process.

FIG. 16 illustrates an uplink multi-user transmission process accordingto another embodiment of the present invention and operations of hiddennodes according to the process. According to the embodiment of FIG. 16 ,a non-legacy PPDU (i.e., HE PPDU) trigger frame 54 is used for the UL-MUtransmission process.

The HE PPDU trigger frame 54 includes a legacy preamble (i.e.,L-Preamble), a non-legacy preamble (i.e., HE-Preamble), and MAC data.The legacy preamble includes L-STF, L-LTF and L-SIG. Also, thenon-legacy preamble includes RL-SIG, HE-SIG-A, HE-SIG-B, HE-STF andHE-LTF. In this case, the RL-SIG, HE-SIG-A and HE-SIG-B are transmittedvia a 64 FFT-based signal. On the other hand, the HE-STF and HE-LTF aretransmitted via a 256 FFT-based signal. The MAC data includes a MACheader and a MAC service data unit (MSDU).

The AP may use the methods described above in FIG. 15 to robustlytransmit the HE PPDU trigger frame 54. In addition, the AP may transmitthe MAC data of the HE PPDU trigger frame 54 in OFDMA form in order toavoid collisions due to OBSS interference in some channels. That is, theMAC data of the HE PPDU trigger frame 54 may be transmitted in duplicatein units of a channel or a subchannel to improve the receptionprobability.

STAs receiving the HE PPDU trigger frame 54 simultaneously transmit anuplink PPDU 60 after an xIFS time. The configuration of the uplinkmulti-user PPDU 60 based on the trigger frame is as described above inFIG. 15 . The AP transmits an M-STA BA to STAs that have succeeded inthe uplink data transmission a SIFS time after the transmission of theUL-MU PPDU 60 is completed. As described above, the length of the M-STABA may vary depending on the number of target STAs of the ACKinformation within the maximum BA length (e.g., d_max_BA).

As described above with reference to FIG. 15 , the remaining timeinformation of the current TXOP may be contained in the preamble of thenon-legacy packet transmitted by the AP and the STA. That is, thenon-legacy preamble of the HE PPDU trigger frame 54 and the UL-MU PPDU60 transmitted in the UL-MU transmission process contains information ofthe remaining TXOP time of the current TXOP. A specific method ofindicating the remaining TXOP time information in the HE PPDU triggerframe 54 is as described above with reference to FIG. 15 . The UL-MUtransmission non-participating terminals that have received at least oneof the HE PPDU trigger frame 54 and the UL-MU PPDU 60 set a NAV based onthe remaining TXOP time information.

The operations of the neighboring terminals L1, H1, L2 and H2 in theUL-MU transmission process of FIG. 16 are as follows. First, thenon-legacy terminal H1 receives the HE PPDU trigger frame 54 transmittedby the AP. According to an embodiment, the terminal H1 may set a NAVbased on duration field information of a MAC header of the HE PPDUtrigger frame 54. The duration field of the MAC header of the HE PPDUtrigger frame 54 indicates a period up to the completion of the M-STA BAtransmission in the corresponding UL-MU transmission process. Accordingto the embodiment of the present invention, the terminal H1 may obtainthe remaining TXOP time information contained in the non-legacy preambleof the HE PPDU trigger frame 54 and set the NAV based thereon.Therefore, the non-legacy terminals receiving the HE PPDU trigger frame54 can set a NAV based on the information of the non-legacy preambleeven in a situation where decoding of the MAC header of thecorresponding packet is unavailable.

Next, the non-legacy terminal H2 receives the UL-MU PPDU 60 transmittedby the STA2. According to an embodiment, the terminal H2 obtains theremaining TXOP time information contained in the non-legacy preamble ofthe corresponding packet and sets a NAV based thereon. According toanother embodiment of the present invention, the terminal H2 may set theNAV by additionally using duration field information of a MAC header ofthe UL-MU PPDU 60.

According to yet another embodiment of the present invention, theterminal H2 may delay the transmission based on the Length field of theL-SIG of the UL-MU PPDU 60 and attempt transmission after a DIFS or anEIFS. If the received packet is a data packet, the terminal shouldfurther wait at least for an EIFS time after a time point based on theLength field. However, if the received packet is an ACK packet, theterminal may further wait for a DIFS time after the time point based onthe Length field. Accordingly, the terminal needs to distinguish whetherthe received packet is a data packet or an ACK packet.

If the received packet is an HE single user (SU) PPDU, the terminal H2can decode the MAC frame of the corresponding packet. Furthermore, ifthe received packet is an HE DL-MU PPDU, the terminal H2 can decode theMAC frame using information extracted from HE-SIG-A and/or HE-SIG-B ofthe corresponding packet. Accordingly, in the above cases, the terminalH2 can distinguish whether the packet is a data packet or an ACK packet.However, if the received packet is an HE UL-MU PPDU, the terminal H2 mayhave difficulty in decoding the MAC header of the corresponding packetwithout information extracted from the trigger frame 54. Therefore,according to a further embodiment of the present invention, an indicatorfor distinguishing whether the packet is a data packet or an ACK packetmay be contained in a legacy preamble or a non-legacy preamble of thenon-legacy packet.

According to an embodiment, the indicator for distinguishing thedata/ACK may be represented by a reserved bit field or an unusedsubcarrier of the legacy preamble. According to another embodiment, theindicator for distinguishing the data/ACK may be represented by a phaserotation, orthogonal sequence of RL-SIG symbols. According to yetanother embodiment, the indicator for distinguishing the data/ACK may berepresented by a combination of modulation schemes applied to twosymbols of the HE-SIG-A. According to a further embodiment of thepresent invention, if it is impossible to distinguish whether thereceived packet is a data packet or an ACK packet, the terminal H2further waits at least for an EIFS time after a time point based on theLength field.

Meanwhile, the operations of the legacy terminal L1 that received the HEPPDU trigger frame 54 and the legacy terminal L2 that received the UL-MUPPDU 60 in the embodiment of FIG. 16 are as described above in FIG. 15 .

FIG. 17 illustrates an uplink multi-user transmission process accordingto yet another embodiment of the present invention and operations ofhidden nodes according to the process. According to the embodiment ofFIG. 17 , a legacy format trigger frame 56 is used for the UL-MUtransmission process. According to an embodiment, the legacy formattrigger frame 56 may be configured as a PPDU in 802.11a format.

The legacy format trigger frame 56 includes a legacy preamble (i.e.,L-Preamble) and MAC data. The legacy preamble includes L-STF, L-LTF andL-SIG. The trigger information of the legacy format trigger frame 56 maybe transmitted via the MAC data of the corresponding frame. When thelegacy format trigger frame 56 is transmitted through multiple channels,the legacy preamble portion of the trigger frame 56 is transmitted induplicate in units of 20 MHz channels. Moreover, since the maximumtransmission bandwidth of a PPDU of the 802.11a format is 20 MHz, theMAC data of the legacy format trigger frame 56 may also be transmittedon a 20 MHz channel basis. According to an embodiment, the MAC data ofthe legacy format trigger frame 56 transmitted on a 20 MHz channel basismay represent the same information in duplicate. According to anotherembodiment of the present invention, in order to shorten the totallength of the trigger frame, the MAC data of the legacy format triggerframe 56 may contain different information for each channel.

STAs receiving the non-legacy format trigger frame 56 simultaneouslytransmit an uplink PPDU 60 after an xIFS time. The configuration of theuplink multi-user PPDU 60 based on the trigger frame is as describedabove in FIG. 15 . The AP transmits an M-STA BA to STAs that havesucceeded in the uplink data transmission a SIFS time after thetransmission of the UL-MU PPDU 60 is completed. As described above, thelength of the M-STA BA may vary depending on the number of target STAsof the ACK information within the maximum BA length (e.g., d_max_BA).

The operations of the neighboring terminals L1, H1, L2 and H2 in theUL-MU transmission process of FIG. 17 are as follows. First, thenon-legacy terminal H1 receives the legacy format trigger frame 56transmitted by the AP. According to an embodiment, the terminal H1 mayset a NAV based on duration field information of a MAC header of thelegacy format trigger frame 56. The duration field of the MAC header ofthe legacy format trigger frame 56 indicates a period up to thecompletion of the M-STA BA transmission in the corresponding UL-MUtransmission process. According to another embodiment, the L-SIG of thelegacy format trigger frame 56 may contain the above-described TXOPduration field in preparation for the case that neighboring non-legacyterminals cannot decode the MAC header due to robust reception or thelike. The terminal H1 may set a NAV based on the TXOP duration field ofthe L-SIG. According to yet another embodiment, the terminal H1 may seta NAV by combining a length obtained in combination of the Length fieldand the Rate field of the L-SIG of the legacy format trigger frame 56with a length obtained based on the duration field of the MAC header.

Next, the legacy terminal L1 receives the legacy format trigger frame 56transmitted by the AP. According to an embodiment, the terminal L1 mayset a NAV based on the duration field information of the MAC header ofthe legacy format trigger frame 56. According to another embodiment, theTXOP duration field may be contained in the L-SIG of the legacy formattrigger frame 56 as described above. In this case, the terminal L1 mayset the NAV based on the TXOP duration field of the L-SIG.

Meanwhile, the operations of the non-legacy terminal H2 and the legacyterminal L2, which have received the UL-MU PPDU 60 in the embodiment ofFIG. 17 , are as described above in FIGS. 15 and 16 .

FIG. 18 illustrates a hidden node protection method in a multi-usertransmission process. In the uplink/downlink multi-user transmissionprocess, NAV setting of UEs not participating in data transmission isrequired. In particular, when multi-user transmission is performed on asubchannel basis, there is a need for a method that enables legacyterminals that cannot receive subchannel data to correctly set the NAV.

A multi-user RTS (MU-RTS) may be transmitted for data transmissionprotection in a multi-user transmission process. The MU-RTS designates aplurality of receivers via a plurality of receiver address fields. Thereceivers receiving the MU-RTS simultaneously transmit CTSs after aSIFS. In this case, the CTSs simultaneously transmitted by a pluralityof receivers have the same waveform. According to an exemplaryembodiment, any one of the same MCS as the MU-RTS, a basic MCS of thecorresponding BSS, or an MCS specified by the MU-RTS may be applied tothe CTS. Also, the same sequence as the MU-RTS is used for the initialsequence applied to the scrambling technique. In the 1-to-ncommunication process between the AP and the plurality of STAs, theneighboring terminals set a NAV based on the duration field value of theMAC header of the MU-RTS and the CTS corresponding thereto.

FIG. 18(a) shows a hidden node protection method in the DL-MUtransmission process. First, the AP transmits an MU-RTS for NAV settingin the DL-MU transmission process. The duration field of the MU-RTS isset to a period up to the end of the DL-MU transmission process. Thatis, the duration field of the MU-RTS frame is set based on a perioduntil the downlink data transmission of the AP and the ACK transmissionsof the STAs are completed. The neighboring terminals of the AP set a NAVuntil the end of the DL-MU transmission process based on the durationfield of the MU-RTS transmitted by the AP. In the embodiment of FIG.18(a), STA1 and STA2 are designated as receivers of the MU-RTS. Thereceiver of the MU-RTS may indicate the receiver of the DL-MUtransmission process.

The receivers receiving the MU-RTS from the AP, that is, STA1 and STA2simultaneously transmit the CTS. The simultaneous CTS transmitted by aplurality of STAs have the same waveform. The duration field of thesimultaneous CTS is set to a period up to the end of the DL-MUtransmission process based on the information of the duration field ofthe MU-RTS. That is, the duration field of the simultaneous CTS is setbased on the period until the downlink data transmission of the AP andthe ACK transmission of the STAs are completed. In FIG. 18(a), theneighboring terminals of STA1 and STA2 set a NAV until the end of theDL-MU transmission process based on the duration field of the CTS.

According to an embodiment of the present invention, the MU-RTS and thesimultaneous CTS may be transmitted on a 20 MHz channel basis.Therefore, the neighboring terminals including the legacy terminals canreceive the MU-RTS and/or the simultaneous CTS and set a NAV. The APtransmits DL-MU data when receiving the CTS from at least one of STA1and STA2, which are receivers of the MU-RTS. In other words, the APtransmits DL-MU data to STA1 and STA2, respectively. The STAs receivethe DL-MU data transmitted by the AP and transmit a multiplexed ACK,i.e., M-STA BA in response thereto.

FIG. 18(b) shows an embodiment of a hidden node protection method in theUL-MU transmission process. According to the embodiment of the presentinvention, a protection method similar to the DL-MU transmission processdescribed above may be used in the UL-MU transmission process.

First, the AP transmits an MU-RTS for NAV setting in the UL-MUtransmission process. The duration field of the MU-RTS is set to aperiod up to the end of the UL-MU transmission process. That is, theduration field of the MU-RTS frame is set based on a period until theuplink data transmissions of the STAs and the M-STA BA transmission ofthe AP are completed. The neighboring terminals of the AP set a NAVuntil the end of the DL-MU transmission process based on the durationfield of the MU-RTS transmitted by the AP. In the embodiment of FIG.18(b), STA1 and STA2 are designated as receivers of the MU-RTS. Thereceiver of the MU-RTS may indicate the transmitter of the UL-MUtransmission process.

The receivers receiving the MU-RTS from the AP, that is, STA1 and STA2simultaneously transmit the CTS. The duration field of the simultaneousCTS is set to a period up to the end of the UL-MU transmission processbased on the information of the duration field of the MU-RTS. That is,the duration field of the simultaneous CTS is set based on the perioduntil the uplink data transmissions of the STAs and the M-STA BAtransmission of the AP are completed. In FIG. 18(b), the neighboringterminals of STA1 and STA2 set a NAV until the end of the UL-MUtransmission process based on the duration field of the CTS.

The AP transmits a trigger frame when receiving the CTS from at leastone of STA1 and STA2, which are receivers of the MU-RTS. The triggerframe may contain resource allocation information for STA1 and STA2,which are receivers of the MU-RTS. The STAs receive the trigger frametransmitted by the AP and transmit uplink multi-user data in responsethereto. The AP receives the uplink multi-user data transmitted by theSTAs and transmits an M-STA BA in response thereto.

FIG. 18(c) shows another embodiment of a hidden node protection methodin the UL-MU transmission process. According to another embodiment ofthe present invention, the AP may transmit a frame in which the MU-RTSand the trigger information are aggregated. For example, the AP maytransmit an MU-RTS configured in a format of a trigger frame. The AP mayindicate that the corresponding frame is an MU-RTS frame through apredetermined field of the trigger frame. The predetermined field is afield indicating the type of the trigger frame.

STAs receiving the MU-RTS in a trigger frame format from the AP transmituplink multi-user data in response thereto. The uplink multi-user datatransmitted by the STAs may contain the simultaneous CTS informationdescribed above. In this manner, the MU-RTS and the trigger frame areaggregated and transmitted, and the CTS and the uplink multi-user dataare aggregated and transmitted, so that the overall time required forthe UL-MU transmission process can be shortened.

FIG. 19 illustrates various embodiments of an MPDU format of an MU-RTS.

First, FIG. 19(a) shows a comparison of formats of a legacy PPDU, thatis, an 802.11a/g PPDU and a non-legacy PPDU, that is, an 802.11ax PPDU.As illustrated, the PPDU consist of a PHY preamble and a MAC ProtocolData Unit (MPDU). As described above, the PHY preamble of the legacyPPDU includes the legacy preamble, i.e., L-STF, L-LTF, and L-SIG. Inaddition, the PHY preamble of the non-legacy PPDU further includes thenon-legacy preamble in addition to the legacy preamble.

FIG. 19(b) shows an embodiment of a configuration of an MPDU. Asillustrated, the MPDU includes a frame control field, a duration/IDfield, address fields (e.g., Address 1 to 3, Address 4), a sequencecontrol field, a QoS control field, an HT control field and a MACService Data Unit (MSDU). The MSDU is determined to have a variablelength, and the end of the MPDU includes an FCS for error checking. Theframe control field includes a type field and a subtype field. The typefield indicates either a control frame, a management frame or a dataframe, and the subtype field indicates either an RTS, a CTS, an ACK or aBA when the type field is indicated as a control frame. The duration/IDfield indicates a value for NAV setting of terminals.

FIG. 19(c) shows an MPDU configuration of an RTS and a CTS among thecontrol frames. In the frame control field of the RTS, the type fieldvalue is indicated as a control frame and the subtype field value isindicated as an RTS, respectively. The duration field indicates lengthinformation for NAV setting. RA indicates a MAC address of the receiverof the RTS, and TA indicates a MAC address of the transmitter of theRTS. According to an embodiment, the RTS may only be transmitted withfixed fields and length.

The terminal receiving the RTS transmits a CTS when the RA value of theRTS matches the MAC address of the corresponding terminal. If the RAvalue does not match the MAC address of the corresponding terminal, theterminal checks an FCS after 2+2+6+6=16 bytes. If the FCS check issuccessful, the terminal sets a NAV based on the duration field of theRTS. However, if the FCS check fails, the terminal may access thechannel again an EIFS after the corresponding PPDU. When the terminaltransmits the CTS, the RA of the CTS is determined as the TA value ofthe RTS. In addition, the duration field value of the CTS is determinedby subtracting the length of the SIFS and the CTS from the durationfield value of the RTS.

FIG. 19(d) shows a method of an MPDU configuration of an MU-RTSaccording to an embodiment of the present invention. In the framecontrol field of the MU-RTS, the type field value is indicated as acontrol frame and the subtype field value is indicated as an RTS,respectively. In the RA field, a plurality of receiver addresses RA_1 toRA are represented in a partial AID (PAID) or a group ID (GID) format.In this case, the addresses of all the STAs participating in themulti-user transmission may not be fully represented, but sufficient NAVsetting effect may occur when the CTS is transmitted from a certainnumber of STAs. According to an embodiment, particular informationindicating that the corresponding MPDU is an MU-RTS may be additionallyrepresented in a frame control field or a PHY preamble.

FIG. 19(e) shows a method of an MPDU configuration of an MU-RTSaccording to another embodiment of the present invention. In the framecontrol field of the MU-RTS, the type field value is indicated as acontrol frame and the subtype field value is indicated as an RTS,respectively. In case of legacy terminals, it is identified that theMPDU is terminated after an FCS of the conventional RTS. According tothe embodiment of the present invention, n receiver addresses RA_1, . .. , RA_n may be further inserted into the MU-RTS after the FCS of theconventional RTS is terminated. In this case, the addresses of all theSTAs participating in the multi-user transmission may be represented bythe respective receiver addresses. According to an embodiment, an ‘n’field may be further included to indicate the number of STAsparticipating in the multi-user transmission. However, according toanother embodiment of the present invention, the ‘n’ field may beomitted when the number of STAs participating in the multi-usertransmission can be inferred from length information of the PHYpreamble. As described above, particular information indicating that thecorresponding MPDU is an MU-RTS may be additionally represented in theframe control field or the PHY preamble. At the end of the MU-RTS, aseparate FCS for the MU-RTS may be inserted.

FIG. 19(f) shows a method of an MPDU configuration of an MU-RTSaccording to yet another embodiment of the present invention. In theframe control field of the MU-RTS, the type field value is indicated asa control frame and the subtype field value is indicated as an MU-RTS,respectively. Also, n receiver addresses RA_1, . . . , RA_n may befurther inserted into the MU-RTS as in the above embodiment. Since thelegacy terminals cannot identify the MU-RTS subfield, the legacyterminals check an FCS corresponding to 4 bytes at the end of thecorresponding MPDU and set a NAV based on the duration field value onlywhen the FCS check is successful.

The embodiments of FIG. 19 illustrate an embodiment of the presentinvention configuring the MU-RTS, and the present invention is notlimited thereto. As described above, the MU-RTS may also be configuredin a trigger frame format.

FIG. 20 illustrates a method of supporting data transmission/receptionof an outdoor STA using MU-RTS and CTS. In the datatransmission/reception of the outdoor STA located far away from the APand located in the open space, the change of the channel delay time issignificant. Therefore, for the outdoor STA, a separate configurationsuch as repetition of signaling information in the preamble portion andlong CP usage in the data portion is required. In the embodiment of thepresent invention, a PPDU having such a separate configuration isreferred to as a non-legacy outdoor PPDU.

According to the embodiment of the present invention, the outdoor STAmay be identified through a transmission process of an MU-RTS and a CTS,and the non-legacy outdoor PPDU may be transmitted. The time it takesfor the AP to transmit the MU-RTS and receive the corresponding CTS fromthe STA is SIFS+2*PD_n. Here, PD_n denotes the propagation delay timebetween an AP and a STA_n, and it is assumed that the time taken in bothdirections is the same. In this case, the outdoor STA among the STAstransmitting the CTS can be determined as follows.

First, when 2*PD_n is equal to or greater than a preset threshold value,the STA_n may be determined as an outdoor STA at a distance from the AP.Second, when the difference value of propagation delay time until thetransmitted CTS has been reached by arbitrary STA_x and STA_y, that is,2*(PD_x−PD_y) is equal to or greater than a preset threshold value, theSTA having a larger propagation delay time may be determined as a doorSTA. According to another embodiment, a STA performing a response to anon-legacy outdoor PPDU may be determined as an outdoor STA. The APtransmits data to the determined outdoor STA via a non-legacy outdoorPPDU.

FIG. 21 illustrates a further embodiment of a downlink multi-usertransmission process. According to the embodiment of the presentinvention, DL-MU transmission can be performed a plurality of timeswithin the same TXOP. In the embodiment of FIG. 21 , an AP transmitsdownlink multi-user data to STA1 and STA2 in the first DL-MUtransmission process, and the AP transmits downlink multi-user data toSTA3 and STA4 in the subsequent DL-MU transmission process. In theembodiment of FIG. 21 , duplicative description of parts which are thesame as or corresponding to the above-described embodiment of FIG. 18(a)will be omitted.

First, referring to the embodiment of FIG. 21(a), when a plurality ofDL-MU transmissions are performed in the same TXOP, transmission andreception of MU-RTS and CTS for the target STAs in the entire DL-MUtransmission process may be performed at once in the initial stage. Thatis, the AP transmits an MU-RTS by designating STA1 and STA2 in the firstDL-MU transmission process and STA3 and STA4 in the second DL-MUtransmission process as receivers. According to an embodiment, theduration field of the MU-RTS is set to a period up to the end of theentire DL-MU transmission process, that is, the end of the first DL-MUtransmission process and the second DL-MU transmission process.Receivers receiving the MU-RTS from the AP, that is, STA1, STA2, STA3and STA4 simultaneously transmit a CTS.

The AP receives the CTS from the STAs and performs a DL-MU transmission.First, in the first DL-MU transmission process, the AP transmits DL-MUdata to STA1 and STA2, respectively. STA1 and STA2 receive the DL-MUdata transmitted by the AP, and transmit an M-STA BA in responsethereto. Next, the AP performs the second DL-MU transmission processwithout a separate backoff procedure. In the second DL-MU transmissionprocess, the AP transmits DL-MU data to STA3 and STA4, respectively.STA3 and STA4 receive the DL-MU data transmitted by the AP, and transmitan M-STA BA in response thereto. According to the embodiment of FIG.21(a), when a plurality of DL-MU transmissions are performed in the sameTXOP, the time for exchange of MU-RTS and CTS can be shortened. However,neighboring terminals of STA3 and STA4 may set a NAV even in the firstDL-MU transmission process for STA1 and STA2, which may unnecessarilyrestrict channel access. Likewise, neighboring terminals of STA1 andSTA2 may set a NAV even in the second DL-MU transmission process forSTA3 and STA4, which may unnecessarily restrict channel access.

Next, Referring to the embodiment of FIG. 21(b), when a plurality ofDL-MU transmissions are performed in the same TXOP, the transmission andreception of MU-RTS and CTS for the target STAs of the correspondingDL-MU transmission process may be performed at the beginning of eachDL-MU transmission process. That is, the AP transmits the first MU-RTSby designating STA1 and STA2 as receivers before DL-MU data transmissionin the first DL-MU transmission process. According to an embodiment, aduration field of the first MU-RTS is set to a period up to the end ofthe first DL-MU transmission process. Receivers receiving the firstMU-RTS from the AP, that is, STA1 and STA2 set a duration field of theCTS based on the duration field of the first MU-RTS, and then transmitthe CTS simultaneously. The AP receives the CTS from the STAs andtransmits DL-MU data to STA1 and STA2. STA1 and STA2 receive the DL-MUdata transmitted by the AP, and transmit an M-STA BA in responsethereto.

The AP receiving the M-STA BA in the first DL-MU transmission processtransmits an MU-RTS for the second DL-MU transmission process without aseparate backoff procedure. That is, the AP transmits the second MU-RTSby designating STA3 and STA4 as receivers before DL-MU data transmissionin the second DL-MU transmission process. According to an embodiment, aduration field of the second MU-RTS is set to a period up to the end ofthe second DL-MU transmission process. Receivers receiving the secondMU-RTS from the AP, that is, STA3 and STA4 set a duration field of theCTS based on the duration field of the second MU-RTS, and then transmitthe CTS simultaneously. The AP receives the CTS from the STAs andtransmits DL-MU data to STA3 and STA4. STA3 and STA4 receive the DL-MUdata transmitted by the AP, and transmit an M-STA BA in responsethereto. According to the embodiment of FIG. 21(b), the neighboringterminals of STA3 and STA4 do not set a NAV in the first DL-MUtransmission process for STA1 and STA2, thereby unnecessary NAV settingof neighboring terminals can be reduced. Likewise, the neighboringterminals of STA1 and STA2 do not set a NAV in the second DL-MUtransmission process for STA3 and STA4, thereby unnecessary NAV settingcan be reduced.

FIG. 22 illustrates a further embodiment of an uplink multi-usertransmission process. According to the embodiment of the presentinvention, a plurality of UL-MU transmissions may be performed withinthe same TXOP. In the embodiment of FIG. 22 , STA1 and STA2 transmituplink multi-user data to an AP in the first UL-MU transmission process,and STA3 and STA4 transmit uplink multi-user data to the AP in thesubsequent second UL-MU transmission process. In the embodiment of FIG.22 , duplicative description of parts which are the same as orcorresponding to the embodiments of FIGS. 18(b) and 18(c) describedabove will be omitted.

First, referring to the embodiment of FIG. 22(a), when a plurality ofUL-MU transmissions are performed in the same TXOP, transmission andreception of MU-RTS and CTS for target STAs in the entire UL-MUtransmission process may be performed at once in the initial stage. Thatis, the AP transmits an MU-RTS by designating STA1 and STA2 in the firstUL-MU transmission process and STA3 and STA4 in the second UL-MUtransmission process at all as receivers. According to an embodiment,the duration field of the MU-RTS is set to a period up to the end of theentire UL-MU transmission process, that is, the end of the first UL-MUtransmission process and the second UL-MU transmission process.Receivers receiving the MU-RTS from the AP, that is, STA1, STA2, STA3and STA4 simultaneously transmit a CTS.

The AP receives the CTS from the STAs and transmits a trigger frame fora UL-MU transmission. First, in the first UL-MU transmission process,the AP indicates STA1 and STA2 to transmit UL-MU data by transmitting atrigger frame. STA1 and STA2 receive the trigger frame transmitted bythe AP and transmit UL-MU data in response thereto. The AP receiving theUL-MU data from the STAs transmits an M-STA BA in response thereto.Next, the AP transmits a trigger frame for the second UL-MU transmissionprocess without a separate backoff procedure. In the second UL-MUtransmission process, the AP indicates STA3 and STA4 to transmit UL-MUdata by transmitting a trigger frame. STA3 and STA4 receive the triggerframe transmitted by the AP, and transmit UL-MU data in responsethereto. The AP receiving the UL-MU data from the STAs transmits anM-STA BA in response thereto.

Meanwhile, according to the embodiment of the present invention, when anadditional UL-MU transmission process is performed within the remainingTXOP time of the same TXOP, an M-STA BA of the previous UL-MUtransmission process and a trigger frame of the next UL-MU transmissionprocess may be aggregated and transmitted in a single A-MPDU. That is,the M-STA BA of the first UL-MU transmission process and the triggerframe of the second UL-MU transmission process may be aggregated andtransmitted in a single A-MPDU.

Next, referring to the embodiment of FIG. 22(b), when a plurality ofUL-MU transmissions are performed in the same TXOP, transmission andreception of MU-RTS and CTS for the STAs in the corresponding UL-MUtransmission process may be performed at the beginning of each UL-MUtransmission process. That is, the AP transmits the first MU-RTS bydesignating STA1 and STA2 as receivers before transmitting a triggerframe for the first UL-MU transmission process. When the first UL-MUtransmission process is completed, the AP transmits a trigger frame forthe second UL-MU transmission process without a separate backoffprocedure. As described above, according to the embodiment of thepresent invention, the M-STA BA of the first UL-MU transmission processand the trigger frame of the second UL-MU transmission process may beaggregated and transmitted in a single A-MPDU.

FIG. 23 illustrates an uplink multi-user transmission process accordingto a further embodiment of the present invention and operations ofhidden nodes according to the process. In the embodiment of FIG. 23 ,the trigger frame 54 according to the embodiment of FIG. 16 describedabove is used, and an MU-RTS 72 and a CTS 74 may be additionally used inthe UL-MU transmission process. In the embodiment of FIG. 23 ,duplicative description of parts which are the same as or correspondingto the above-described embodiment of FIG. 16 will be omitted.

According to the embodiment of the present invention, when at least oneof the following predetermined conditions is satisfied in the UL-MUtransmission process, the MU-RTS 72 and the CTS 74 may be transmitted inadvance. 1) the HE PPDU trigger frame 54 is used for the UL-MUtransmission process, 2) the length of UL-MU data is a predeterminedlength or more, 3) the number of STAs participating in the UL-MUtransmission process is a predetermined number or more, 4) the length ofan M-STA BA in the UL-MU transmission process is expected to increasebeyond a certain length, 5) there is a restriction of the format or MCSof the PPDU to be used for the M-STA BA in the UL-MU transmissionprocess, 6) the UL-MU data is transmitted through a wideband channel orusing an OFDMA, and 7) at least one outdoor STA is present among STAsparticipating in the UL-MU transmission.

Meanwhile, although the conditions for use of the MU-RTS 72 and the CTS74 in the UL-MU transmission process are described above, the MU-RTS 72and the CTS 74 may be additionally used in the DL-MU transmissionprocess as well in a similar manner.

Although the present invention is described by using the wireless LANcommunication as an example, the present invention is not limitedthereto and the present invention may be similarly applied even to othercommunication systems such as cellular communication, and the like.Further, the method, the apparatus, and the system of the presentinvention are described in association with the specific embodiments,but some or all of the components and operations of the presentinvention may be implemented by using a computer system having universalhardware architecture.

The detailed described embodiments of the present invention may beimplemented by various means. For example, the embodiments of thepresent invention may be implemented by a hardware, a firmware, asoftware, or a combination thereof.

In case of the hardware implementation, the method according to theembodiments of the present invention may be implemented by one or moreof Application Specific Integrated Circuits (ASICSs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), processors, controllers, micro-controllers, micro-processors,and the like.

In case of the firmware implementation or the software implementation,the method according to the embodiments of the present invention may beimplemented by a module, a procedure, a function, or the like whichperforms the operations described above. Software codes may be stored ina memory and operated by a processor. The processor may be equipped withthe memory internally or externally and the memory may exchange datawith the processor by various publicly known means.

The description of the present invention is used for exemplification andthose skilled in the art will be able to understand that the presentinvention can be easily modified to other detailed forms withoutchanging the technical idea or an essential feature thereof. Thus, it isto be appreciated that the embodiments described above are intended tobe illustrative in every sense, and not restrictive. For example, eachcomponent described as a single type may be implemented to bedistributed and similarly, components described to be distributed mayalso be implemented in an associated form.

The scope of the present invention is represented by the claims to bedescribed below rather than the detailed description, and it is to beinterpreted that the meaning and scope of the claims and all the changesor modified forms derived from the equivalents thereof come within thescope of the present invention.

INDUSTRIAL APPLICABILITY

Various exemplary embodiments of the present invention have beendescribed with reference to an IEEE 802.11 system, but the presentinvention is not limited thereto and the present invention can beapplied to various types of mobile communication apparatus, mobilecommunication system, and the like.

1-16. (canceled)
 17. A wireless communication terminal comprising: atransceiver configured to transmit and receive wireless signals; and aprocessor configured to process wireless signals transmitted or receivedthrough the transceiver, wherein the processor is configured to:transmit an uplink PPDU(Physical Layer Protocol Data Unit) to anAP(Access Point), and receive an A-MPDU(aggregate MPDU) in response tothe uplink PPDU, wherein the A-MPDU includes a trigger frame solicitingtransmission of a next uplink PPDU and an acknowledgment frame for oneor more PPDUs, including the uplink PPDU, transmitted from one or moreterminals triggered by the AP, and wherein the trigger frame is includedin the A-MPDU by being aggregated with the acknowledgement frame upon aprocess for the transmission of a next uplink PPDU being performedwithin a current TXOP(Transmission Opportunity).
 18. The wirelesscommunication terminal of claim 17, wherein at least one non-legacypreamble of the uplink PPDU, the trigger frame, or the next uplink PPDUincludes remaining TXOP(Transmission Opportunity) time information of acurrent TXOP.
 19. The wireless communication terminal of claim 18,wherein the remaining TXOP time information is indicated by apredetermined TXOP duration field of a High Efficiency Signal fieldA(HE-SIG-A) of the non-legacy preamble.
 20. The wireless communicationterminal of claim 19, wherein a number of bits in the TXOP durationfield is less than a number of bits in the duration field indicating theremaining TXOP time information in the MAC header of the correspondingPPDU.
 21. The wireless communication terminal of claim 17, wherein theacknowledgement frame is located before the trigger frame in the A-MPDU.22. The wireless communication terminal of claim 17, wherein theacknowledgement frame includes a multi-STA block ACK having a variablelength within a maximum length of a block response.
 23. The wirelesscommunication terminal of claim 17, wherein each of one or more bitsconstituting the acknowledgment frame indicates an Ack or Nack for acorresponding a PPDU among the one or more PPDUs.
 24. A wirelesscommunication method of a wireless communication terminal comprising:transmitting an uplink PPDU(PHY Protocol Data Unit) to an AP(AccessPoint); and receiving an A-MPDU(aggregate MPDU) in response to theuplink PPDU, wherein the A-MPDU includes a trigger frame solicitingtransmission of a next uplink PPDU and an acknowledgment frame for oneor more PPDUs, including the uplink PPDU, transmitted from one or moreterminals triggered by the AP, and wherein the trigger frame is includedin the A-MPDU by being aggregated with the acknowledgement frame upon aprocess for the transmission of a next uplink PPDU being performedwithin a current TXOP(Transmission Opportunity).
 25. The wirelesscommunication method of claim 24, wherein at least one non-legacypreamble of the uplink PPDU, the trigger frame, or the next uplink PPDUincludes remaining TXOP(Transmission Opportunity) time information of acurrent TXOP.
 26. The wireless communication method of claim 25, whereinthe remaining TXOP time information is indicated by a predetermined TXOPduration field of a High Efficiency Signal field A(HE-SIG-A) of thenon-legacy preamble.
 27. The wireless communication method of claim 26,wherein a number of bits in the TXOP duration field is less than anumber of bits in the duration field indicating the remaining TXOP timeinformation in the MAC header of the corresponding PPDU.
 28. Thewireless communication method of claim 24, wherein the acknowledgementframe is located before the trigger frame in the A-MPDU.
 29. Thewireless communication method of claim 24, wherein the acknowledgementframe includes a multi-STA block ACK having a variable length within amaximum length of a block response.
 30. The wireless communicationmethod of claim 24, wherein each of one or more bits constituting theacknowledgment frame indicates an Ack or Nack for a corresponding a PPDUamong the one or more PPDUs.